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Author: johnnyrisk Two stars, 250 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 71152  
Subject: Any Diesel Heads Out There? Date: 2/19/2002 12:51 PM
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I recently changed jobs and now have a long commute. Because of the high mileage that I expect to accumulate, I decided to buy a commuter car that would be good for the rigors of Interstate driving with respect to safety and reliability as well as being fully depreciated. I also wanted at least 30 mpg. The short list of vehiciles that fit these requirements included the early to mid 1980s Mercedes Benz 300D turbodiesel.

Without much seaching I found a 1982 with 173K on it locally, and bought it for less than the tax on a new car. The car is in very good condition except for the suspension which needed shocks and bushings. My only concern is cold starting. I replaced the battery which was on the edge of collapse, but it still is hard to start if the temperature is below 30 F. Above 40 and it starts easily.

I suspect that the glow plugs are original. The owners manual states that you need to hold the accelerator to the floor when it is below feezing and crank away. Above 40 F, the engine starts in about 2 seconds, but takes at least 15 seconds when it is below freezing. Does this sound like the glow plugs are weak? I do not have a manual yet. Does anyone know how to test them in place? What should the resistance to ground be?

Regards,
Johnny Risk

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Author: peplow Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18239 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/19/2002 4:34 PM
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but takes at least 15 seconds when it is below freezing. Does this sound like the glow plugs are weak?...................................


Do you mean 15 seconds of cranking, or 15 seconds for a "glow plug" indicator light or something to go out?


If you think the Glow plugs are original(1982), then replace 'em. A bus company I used to work for, figured that they often needed replacing after about a year, mind you these were in a 7.2 liter International(Ford) engine.






Pete


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Author: johnnyrisk Two stars, 250 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18240 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/19/2002 9:43 PM
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Peplow,

Re Glow plugs

The yellow glow plug indicator goes out in about three seconds or less. The cranking time increases with decreasing temperature. When the temperature is below freezing, I've had to jump the car if it doesn't start on the first attempt. This is what concerns me.

My theory is that if the resistence of the glow plugs has increased over time, the current when the indicator is on is less and, the amount of power consumed also dropped. The end result is a lower temperature in the prechamber leading to more diffuculty starting.

Before replacing the glow plugs, I would like to take resistence measurements. Also, this car lived in Florida until 1997 so it probably didn't even need functioning glow plugs to start.

I have heard that Mercedes glow plugs should be replaced every 100,000 miles, but have not seen this in writing. It just seems strange that there should be such a difference in starting behavior with a 10 degree change in temperaure.

Regards,

Johnny Risk

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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18241 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/19/2002 10:05 PM
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My theory is that if the resistence of the glow plugs has increased over time, the current when the indicator is on is less and, the amount of power consumed also dropped. The end result is a lower temperature in the prechamber leading to more diffuculty starting.


If the resistance has increased the temperature would be higher not lower. Also, the resistance value will change when the temp of the glow plug changes, so a cold resistance check may not indicate a problem even if the plug is bad.

Try cycling the key a number of times before actually cranking the car to allow the glow plugs more time to warm up the combustion chamber.

tjt

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18246 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 10:21 AM
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If the resistance has increased the temperature would be higher not lower.

Temperature is proportional to energy dissipation, which is the time integral of power.

Power = V^2/R. If the resistance increases, the power decreases, hence the energy dissipated decreases and the temperature drops.

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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18247 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 10:44 AM
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Ok jim, I'll bite.

This is an electrical circuit right? Does Ohm's law not apply? I am the first to admit I am not a diesel head, but in every electrical circuit I have ever encountered increased resistance will result in higher temperatures. I mean that is the whole idea behind the design of the glow plug and even the light bulb.

Please enlighten me.

tjt

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18248 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 10:51 AM
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This is an electrical circuit right? Does Ohm's law not apply? I am the first to admit I am not a diesel head, but in every electrical circuit I have ever encountered increased resistance will result in higher temperatures. I mean that is the whole idea behind the design of the glow plug and even the light bulb.



Of course Ohms law applies. I used it. The voltage is fixed (12 volts). Hence, obviously, if the resistance increases, the current decreases (V/R = I). Power goes like resistance and square of current: P=I^2*R. But since I = V/R, then the simple substitution gives P = V^2/R (as I provided in the previous post) hence increasing resistance indicates decreasing power. Energy is integral with time of Power, so decreasing power indicates decreasing energy, hence decreasing temperature.

When the voltage is fixed, of course.

QED.

Please enlighten me.

I already did, in the previous post, and I elaborated in this post. Before coming back at me, please check my profile.

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Author: vuelta Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18249 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:01 AM
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This is an electrical circuit right? Does Ohm's law not apply? I am the first to admit I am not a diesel head, but in every electrical circuit I have ever encountered increased resistance will result in higher temperatures.

Same equation as the one Jim posted, but substituting I for V/R (Ohm's law):

Power = VI

Less current (I) will flow at a higher resistance, given that voltage is fixed.

vuelta



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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18251 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:09 AM
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I already did, in the previous post, and I elaborated in this post. Before coming back at me, please check my profile.

Sorry if I sounded like I was "coming back at you" I was honestly looking for more elaboration. I have checked your profile before and I don't doubt you know what you are talking about.

My confusion lies in the (maybe wrong) assumption that a glow plug works much like a light bulb. The designed resistance in the circuit (filament) causes an increase in temperature which makes the filament white hot and glow. Is this not how a glow plug is designed?

tjt

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Author: SRHCB Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18252 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:13 AM
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Q: How many electrical engineers does it take to fix an old Mercedes Diesel?

A: I don't know. But its got to be more than the number of physics teachers who can dance on the head of a glow plug.

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18253 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:15 AM
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My confusion lies in the (maybe wrong) assumption that a glow plug works much like a light bulb. The designed resistance in the circuit (filament) causes an increase in temperature which makes the filament white hot and glow. Is this not how a glow plug is designed?

Sure that is how a glow plug works. You are confusing constant current and constant voltage.

If the voltage is fixed (as, by a battery), then the current varies with the resistance. Increasing resistance leads to decreasing current. This is the situation both with the glow plugs and with household power supplies.

If the current is fixed, meaning that 1 Amp will be delivered into ANY load, then the voltage varies. In this case, increasing resistance motivates increasing voltage and since power goes as V^2/R, the power increases, hence the temperature increases. You will encounter this situation in certain vehicle sensors (such as the mass airflow sensor), and in a lot of electronic circuits. But you won't find it in household wiring or in glow plug circuits.


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Author: eudaimon6 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18254 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:19 AM
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OK, i was a history major in college, and took exactly one physics class. it was pretty cool.

anyway-isnt that how things like glow plugs and toasters work-the electrical current is converted into heat? if you have minimal resistance at a given voltage, then very little heat is generated. if you have more resistance, then more heat is generated. if the glow plugs had no resistance, then they wouldnt glow-the current would just pass through.

i must be missing something.

http://www.howstuffworks.com/search.php?terms=ELECTRICAL+RESISTANCE

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Author: eudaimon6 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18255 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:21 AM
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that is what thought-the how stuff works website specifically compares a diesel engine glow plug to a light bulb.

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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18256 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:26 AM
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i must be missing something.

Don't worry dean, I'm missing the same thing.

tjt

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18257 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:48 AM
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Don't worry dean, I'm missing the same thing.

This is getting kind of OT, but if no one else objects, we can work on it for a little bit.

Lets try it this way. Ohms law says V=IR.

Now, assume that V is fixed and CANNOT change. In this case, if R increases, what happens to I?

When you answer this, we'll move on to the next step.

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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18258 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:09 PM
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Same thing that happens on the highway when 4 lanes converge into one. The traffic/electrons slow down, so current flow decreases.

However, the kinetic energy those moving electrons has to do something, so it heats up the wire. Unlike vehicular traffic, electrons don't like to merge very well and they end up bumping into each other and generating lots of heat.

I have a prety good understanding of electrical theory as well as practical application in automobiles. You don't have to be condesending to convince me or anyone else you are right. I admit I could be wrong and would be more than happy to learn something even if it means I was wrong.

I can SEE and FEEL what happens to the light bulb, toaster, PTC resistor, etc when the resistance is increased and the temperature DOES increase. You said in an earlier post that a glow plug works much like a light bulb including the constant volotage. I don't dispute the proofs you posted re: power. What I would like to see is the proof that Power=temperature. And if you could prove that, how does the toaster/lightbulb defy that law?

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Author: eudaimon6 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18259 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:10 PM
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ok

v equals 12
so if r equals four then i must equal three

if r increases to six then i must decrease to two

ok. so why does the glow plug get hot? is that the next step, or will there be intermediate ones?

thanks
dean



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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18260 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:25 PM
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v equals 12
so if r equals four then i must equal three

if r increases to six then i must decrease to two


Correct. If voltage is fixed and resistance increases, then current MUST decrease.

Now, the power in a DC circuit is given as the product of voltage and current. Or,

P = VI.

Since V = IR, we can substitute to remove the voltage from the power equation, giving

P = I^2*R.

Now, using the numbers that you used in your previous response, if R goes up, and I goes down accordingly, what happens to P?

ok. so why does the glow plug get hot? is that the next step, or will there be intermediate ones?

It gets hot due to electrical resistance. How hot it gets depends on the energy. Energy is closely related to power. We are getting there.



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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18261 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:28 PM
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Energy is integral with time of Power, so decreasing power indicates decreasing energy, hence decreasing temperature.


Here is the part I think may be wrong. The decreasing energy of the electrons does NOT result in decreasing temperature, but since energy cannot be created or destroyed, that decreasing energy (of the electrons) is transmitted in the form of heat.

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18262 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:29 PM
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Same thing that happens on the highway when 4 lanes converge into one. The traffic/electrons slow down, so current flow decreases.

Analogies are useful, but seldom exact. A better analogy to current flow in a wire is the analogy of water in a pipe. Even so, the analogy is not exact and shouldn't be stretched too far.

I have a prety good understanding of electrical theory as well as practical application in automobiles. You don't have to be condesending to convince me

You have a fundamental misapprehension of an important aspect of electrical theory. It has been my long experience that it is better to guide you to the answer, rather than give you the answer. I have given you the answer in two separate posts now, but you have not apprehended it.

I am sorry that you think I am being condescending; I am not. Rather, I am trying to get you to lead yourself to the answer - and along the way, perhaps learn something.

However, you don't see it that way, so I will respond no further to you on this thread.



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Author: eudaimon6 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18263 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 12:37 PM
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ok

in the previous illustration, we had v=12, i=3 and r=4. we then raised r to 6 and saw that with v held constant i decreased to 2.

plugging into our new equation, we have

36= (3^2)(4)

making the same changes in r and v, we get

24 = (2^2)(6)

so power decreases with resistance increased and voltage held constant.

so the difference must be the heat produced?

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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18264 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 1:27 PM
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However, you don't see it that way, so I will respond no further to you on this thread.


Interesting tactic instead of addressing my specific challenges or admitting you were not 100% correct.

tjt

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Author: SRHCB Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18265 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 2:26 PM
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Coming from an engineering background, (albeit Civil rather than electrical), I will put up a word in "defense" of jiml8 or engineers/scientists/academics in general.

It might be right, and it be workable. It might even be both.

PS: And what was the original question? Oh yeah; I'd suppose I'd replace the glow plugs.

PSII: (an old friend) Milan Lonchar's Second Law of Physics: "If you don't move, nothin else does either."

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18266 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 2:30 PM
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so power decreases with resistance increased and voltage held constant.

Absolutely correct. Now, the energy through the circuit is simply the power through the circuit times the amount of time the power is applied to the circuit; viz:

E = P*t

So, for constant voltage, if the resistance is increased, the power decreases, hence the energy through the circuit decreases.

Now, heat is a form of energy, hence the amount of heat decreases and your glow plug runs colder, not hotter.

so the difference must be the heat produced?

Yep. You got it. To make the plug run hotter, you would decrease its resistance. If you take this too far, the battery cannot deliver enough current and will heat up itself (internal resistance of the battery).

Now, earlier you commented that if the plug had zero resistance (a superconductor) there would be no heating. Let's look at it.

If the plug has zero resistance, then by ohm's law (V=IR), the voltage across the plug MUST BE zero.

But, this plug is connected to a battery. So the battery wants to put 12 volts across that plug. So, the battery will deliver more and more and more current, trying to develop a voltage across the plug.

One of two things will happen; either the battery will deliver a current in excess of the superconductor critical current, at which point the superconductor would revert instantaneously to a normal conductor - with spectacular consequences - or the battery would melt from the heat.

In the other extreme, if the plug had infinite resistance (open circuit) it would pass no current, hence there would be no heating.


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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18267 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 2:31 PM
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Interesting tactic instead of addressing my specific challenges or admitting you were not 100% correct.

I am 100% correct, and I will not debate it.

If you don't wish to learn, that is up to you. I will not waste my time if you won't listen.

It's just that simple.

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Author: eudaimon6 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18268 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 2:57 PM
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ok i think i understand my problem with this discussion

going back to the water analogy.

voltage is like water pressure

amperage is rate of water flow, or electron flow.

resistance is diameter of the water hose

what is power? i dont think i understand the definition of power.

thanks

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Author: TMFTwitty Big funky green star, 20000 posts Old School Fool CAPS All Star Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18269 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 4:10 PM
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what is power? i dont think i understand the definition of power.

hmmmm, maybe power is the energy converted by what you are doing with the water.

Anyway, hope the Fool gets the car started. I'd check for generally equal resistance in all the glow plugs. That being true, I'd either cycle them like someone previously said, or replace them...after checking for high resistance connections, and accurately measuring available voltage at the glow plugs. If voltage is not system at all the plugs, I'd check it at the relay too. Dirty or oxidized relay contacts might not be delivering full voltage.

Twit

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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18270 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 5:07 PM
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voltage is like water pressure

amperage is rate of water flow, or electron flow.

resistance is diameter of the water hose


Exactly. Thinking of it this way is helpful.

what is power? i dont think i understand the definition of power.

Power is the rate of energy usage.



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Author: jiml8 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18271 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 5:13 PM
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Anyway, hope the Fool gets the car started. I'd check for generally equal resistance in all the glow plugs. That being true, I'd either cycle them like someone previously said, or replace them...after checking for high resistance connections, and accurately measuring available voltage at the glow plugs. If voltage is not system at all the plugs, I'd check it at the relay too. Dirty or oxidized relay contacts might not be delivering full voltage.

With the mileage on the vehicle, it's also possible that there is a compression problem, causing hard starts when very cold. Diesels should go 300K miles or so, but you never can tell.


Beyond the glow plugs, it would be a good idea to check the circuit that fires the glow plugs to make sure it isn't turning off too soon. I don't know; is that set by a timer, or is there a sensor monitoring the precombustion chamber temp?

Hmmm...could it be that the current through the glow plugs is being monitored? Would be simple enough. As the glow plugs heat, their resistance should increase a bit, so it should be possible to determine when they reach a set temperature. As the glow plugs age and the element either becomes fouled or oxidized or corroded, its resistance might increase - or, if it's fouled, the carbon deposits would act as an insulator, causing the sensor to turn it off too soon.

I guess I don't know how that circuit works, exactly, but finding out would be a good plan. Maybe just cleaning the glow plugs is all that's needed.




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Author: RandGraham One star, 50 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18272 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 6:37 PM
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Johnny Risk,
Congratulations on purchasing a fine automobile.

To answer your specific questions, yes it sounds like at least one of your glow plugs is out. There is a simple test to check the resistance of your glow plugs. There is a connector and you check with an ohmeter between the connector and ground. All the glow plugs should read the same resistance. This resistance should be ~1 Ohm or less. As the glow plugs fail the resistance gets larger and larger. You can get the manuals on cd from MBUSA ~$99. These are the factory manuals and they include a chassis manual, engine manual, electronic trouble shooting manual, representative owner's manual and a routine maintenance manual. The glow plug check is also in the Haynes manual ~$12.

I have a 1984 300D. I checked all my glow plugs and found that 2 of them were reading infinite resistance. I replaced all 5 at the same time they are about $9 each from Rusty Cullens. Before replacing the glow plugs I had trouble starting with ambient temps around 30 degrees. It required considerable cranking and produced lots of white smoke. Basically, I had similar symptoms to what you are reporting. At least one of your glow plugs may be bad.

Another key to cold starting is to have the valves adjusted. These need to be adjusted every 15000 miles. If you don't have maintenance records showing they have been adjusted, get them adjusted.

As was mentioned compression is key to cold starting. In fact it may be the best compression test. If your car will start in the cold it means that it has good compression and the engine has plenty of life left. As you know the engine in your car can go over 400,000 miles if properly maintained.

Here is Rusty Cullens info:
http://rusty.mbz.org/
I get all my parts from him. I have found that he is the cheapest supplier of oem quality parts. He also knows the cars.

Also, the following mailing list has been a great help to me.
http://lists.mbz.org/diesel/
You can subscribe to the list and or search the archives for just about any topic.

Info on the manuals:
http://www.mbz.org/info/articles/rtfm.html

Here are pics of my car:
http://home.attbi.com/~randgraham/photo_albums/our_cars/index.htm


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Author: peplow Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18273 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 6:44 PM
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The yellow glow plug indicator goes out in about three seconds or less. The cranking time increases with decreasing temperature. When the temperature is below freezing, .......................................

How much below freezing?
Ok, At freezng does it turn over as fast as it does at, say,50 degrees? What,s in it for oil, 15w40? how "good" is the battery?
Do you add a "conditioner" to the fuel ? How clean are battery connections ? Check the relay for the Glow plug circuit ?

I would guess one or more Glow plugs isn't working, and in combination with ANY of the above may be the problem.




Pete

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Author: peplow Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18274 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 6:45 PM
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It just seems strange that there should be such a difference in starting behavior with a 10 degree change in temperaure. .............


Not really, depending on WHICH 10 degrees.


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Author: johnnyrisk Two stars, 250 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18275 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 10:38 PM
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Wow,

My original post generated quite a thread. As an engineer, I have some experience with hetereogenous combustion which is how diesel fuel burns. The thing I am a little fuzzy on is whether the glow plugs act as short term spark plugs or serve to input energy into the fuel/air mixture during startup. I suspect that the glow plugs are simply a means of adding BTUs to the mix and help to offset the heat losses to the cold cylinder walls. If this is true, then increasing resistence results in decreased power and hence decreased energy to the fuel mix.

I took the car in for an alignment today and spoke to the mechanic about the cold starting problem. This garage specializes in European cars and is no stranger to MB diesels.

His opinion is that one or more of the glow plugs are either weak or open. There are five cylinders, so just two malfunctioning glow plugs is significant. This may explain why it takes a long time (15 seconds of cranking) with a lot of missing when the temperature is below freezing.

Anyway I will find out the results tommorow. I will report back when I find out more.

Regards,

Johnny Risk

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Author: RandGraham One star, 50 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18276 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/20/2002 11:08 PM
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The thing I am a little fuzzy on is whether the glow plugs act as short term spark plugs or serve to input energy into the fuel/air mixture during startup


The glow plugs heat the air in the prechamber. MB diesels are an indirect injection diesel. This means they have a prechamber that is attached to the cylinder. The glow plug is in the prechamber. When the key is in the on position the glow plug heats up for about 40 seconds. After about 40 seconds the glow plug no longer heats to avoid damaging the glow plug. Once the key is turned to the start position the glow plug no longer heats. As you know, diesel ignition happens due to the compression in the cylinder. When ambient temperatures are cold the glow plugs heat up the air to allow combustion to occur. After that the cylinder is hot enough for combustion to occurr normally.

Also note in later model mercedes diesels, say cars built in the 1990s, the glow plugs stay on after the car is running. This is to make the combustion more efficient and to reduce emissions. In these cars the glow plugs fail more frequently and need to be replaced more often becuase they are on more of the time.


His opinion is that one or more of the glow plugs are either weak or open.

Well if he is a mechanic familiar with MB diesels, he will know how to test the glow plugs. Should take 10-15 minutes to check the glow plugs.

Fool on,
Rand

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Author: rsprang Big gold star, 5000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18279 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 8:29 AM
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If the resistance has increased the temperature would be higher not lower.

If we assume a constant voltage across the glow plug, and ohms law:
V=IR

is becomes apparent that if resistance increases, current must decrease.

The power consumption (eg essentially the heat) formula is :

P = VI

so, if current decreases and voltage is constant (and resistance increases), the heat output decreases.

In other words, the temperature will decrease as the resistance increases.

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Author: theocat Big red star, 1000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18280 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 11:37 AM
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Hi everyone:

May I weigh in on this?

For one, regardless of how great MB diesels are, we're talking about a 179,000 mile CAR, which has brakes, suspension, exhaust, electrical systems, cooling, and the proposed use is a high mileage commuter. I'd say pass this one by. That's too many miles in MHO for the intended prupose. It might make a decent secondary car, but all contraptions mechanical have a useful lifespan, from Der Fatherland or not.

Unless you got the thing for $500 bucks, and you know the ball joints and tie rod ends , etc[any part that can kill ya]are brand spankin' new.

Part Two: Resistance varies with temperature for most materials, and some materials actually show a DECREASE in R with INCREASING T. I know for a fact Carbon is one such material. I wonder what material the glow plug head is made of.[just adding a twist to the discussion].

Theo


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Author: rsprang Big gold star, 5000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18281 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 12:23 PM
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The thing I am a little fuzzy on is whether the glow plugs act as short term spark plugs or serve to input energy into the fuel/air mixture during startup

The glow plugs are heated initially to start the reaction, but the heat of combustion keeps them hot after that, when the engine is running.

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Author: 0x6a74 Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18282 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 12:56 PM
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If the resistance has increased the temperature would be higher not lower.

//////////////
In other words, the temperature will decrease as the resistance increases.


i guess i'm missing something.... but those sound like
exact opposite, and
yet everyone is 100% certain, And using same equations.

(Just curious......

and/or splain this:
Lamp -- power cord: relatively thick, mostly cool
-- light bulb filament: relatively thin, seriously hot...
isn't the filament the part of that circuit with highest
resistance?

which *seems* to imply, increase in resistance 'cause' increase in
temp.
)



-jp

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Author: rsprang Big gold star, 5000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18283 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 1:04 PM
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and/or splain this:
Lamp -- power cord: relatively thick, mostly cool
-- light bulb filament: relatively thin, seriously hot...
isn't the filament the part of that circuit with highest
resistance?


Yes, the filament has higher resistance than copper wire, by design. And higher wattage bulbs have lower resistance, and are thus brighter.


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Author: grue22 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18284 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 1:29 PM
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yet everyone is 100% certain, And using same equations.

I've been following along, and I haven't found anything that jiml8 has said that was incorrect*, so I haven't commented. As the glow plug gets hotter, its resistance goes up, and the power disspated in it goes down. Eventually, the power going into the plug equals the heat being conducted away from it and an equilibrium temperature is reached.

Power dissipated in the component, P = V²/R, so for constant voltage, power goes up as resistance goes down.


and/or splain this:
Lamp -- power cord: relatively thick, mostly cool
-- light bulb filament: relatively thin, seriously hot...
isn't the filament the part of that circuit with highest
resistance?


Yup, filament is the highest resistance in the circuit. Since all the components are in series, the current through each piece is the same. When the current is fixed, the power deposited in each piece is proportional to its resistance. (P = I²R) Therefore, the cord stays cool and the filament gets hot enough to glow. (We'd be really upset if it were the other way around.)

Clear?

Grue


* In case I need credentials - 14 years of college as a student, researcher, and teacher - BS and PhD in mechanical engineering - I know this field, in fact I used to teach in this area, now let's see how effective I am at explaining it on a discussion board.

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Author: rsprang Big gold star, 5000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18285 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 1:40 PM
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Yup, filament is the highest resistance in the circuit. Since all the components are in series, the current through each piece is the same. When the current is fixed, the power deposited in each piece is proportional to its resistance. (P = I²R) Therefore, the cord stays cool and the filament gets hot enough to glow.

That seems to contradict what jiml8 (and I) posted. A glow plug circuit is also a series circuit, but jiml8 said heat decreases as resistance increases, due to the fixed voltage. A single light bulb is the same situation - the voltage across the bulb is essentially constant, so as the resistance increases, the current drops, and less power is dissipated.

I think the issue is that you assumed the current was fixed. The current isn't fixed, the voltage is. The current is set by the resistance.


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Author: 0x6a74 Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18286 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 1:45 PM
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Power dissipated in the component, P = V²/R, so for constant voltage, power goes up as resistance goes down.
//////////////
and/or splain this:
Lamp -- power cord: relatively thick, mostly cool
-- light bulb filament: relatively thin, seriously hot...
isn't the filament the part of that circuit with highest
resistance?
//////////

Yup, filament is the highest resistance in the circuit. Since all the components are in series, the current through each piece is the same. When the current is fixed, the <p>power deposited in each piece is proportional to its resistance. (P = I²R) Therefore, the cord stays cool and the filament gets hot enough to glow. (We'd be really upset if it were the other way around.)

Clear?


no.... but i'm maybe closer to seeing why i'm confused...
seems in one case Voltage is constant, and in the other case, somehow
Amperage....

but ? if V = I*R...... if R increases, no way Voltage and Current Both constant ?


-jp


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Author: grue22 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18287 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 2:01 PM
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Yup, filament is the highest resistance in the circuit. Since all the components are in series, the current through each piece is the same. When the current is fixed, the power deposited in each piece is proportional to its resistance. (P = I²R) Therefore, the cord stays cool and the filament gets hot enough to glow.

>>>>>>>>>>>>>>>>>>>>>>>>

That seems to contradict what jiml8 (and I) posted. A glow plug circuit is also a series circuit, but jiml8 said heat decreases as resistance increases, due to the fixed voltage. A single light bulb is the same situation - the voltage across the bulb is essentially constant, so as the resistance increases, the current drops, and less power is dissipated.

I think the issue is that you assumed the current was fixed. The current isn't fixed, the voltage is. The current is set by the resistance.



What I *tried* to say was correct, but I see now that the way that I said it was unclear. What I was trying to point out was that since the current through each component in the circuit is the same, the energy will be deposited preferentially in the component with the highest resistance.

You are correct. The current in the circuit is determined by the total resistance of the circuit and the voltage applied. When I said that the current was "fixed" in the statement above, I meant that it was fixed as a constant parameter in the equations (the same value for each component and not varying with respect to time), not fixed for the circuit regardless of resistance.

Grue

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Author: grue22 Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18288 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 2:10 PM
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no.... but i'm maybe closer to seeing why i'm confused...
seems in one case Voltage is constant, and in the other case, somehow
Amperage....

but ? if V = I*R...... if R increases, no way Voltage and Current Both constant ?


You are correct, and my statement was confusing. For all of these circuits, the voltage is fixed at around 12 volts, and the current is determined by the circuit voltage.

However, the current through each element of the series circuit is identical. So the value for the current is "fixed" in that it does not vary from component to component in a given circuit, but its value changes from circuit to circuit.

Did that help, or did I further confuse you?

Grue
... I'd rather have a blackboard and see your face as I try to explain

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Author: mind2know Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18289 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 2:30 PM
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This may be a usefull link...

http://www.dieselpage.com/tip1.htm

Rein.

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Author: Hyperborea Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18291 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 3:29 PM
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Here is Rusty Cullens info:
http://rusty.mbz.org/
I get all my parts from him. I have found that he is the cheapest supplier of oem quality parts. He also knows the cars.


I will second that recommendation. I have dealt with Rusty for Porsche parts and he has provided great service at a good price.

Hyperborea

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Author: RandGraham One star, 50 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18296 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 5:00 PM
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A glow plug circuit is also a series circuit,


What do you mean by this? Are you speaking in general terms? Are we still talking about Mercedes W123.xxx chassis cars with engine 617.xxx? Since 1980 the glow plugs in MB diesel engines have been wired in parallel.

Regards,
Rand

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Author: rsprang Big gold star, 5000 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18298 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 5:06 PM
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A glow plug circuit is also a series circuit,


What do you mean by this? Are you speaking in general terms? Are we still talking about Mercedes W123.xxx chassis cars with engine 617.xxx? Since 1980 the glow plugs in MB diesel engines have been wired in parallel.


Didn't know that - I'm not familiar with diesels. As long as each glow plug has essentially the same resistance, however, they will act like one resistor of the equivalent value if they are wired in parallel.


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Author: peplow Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18309 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 5:57 PM
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Does this sound like the glow plugs are weak? ........................

Unless the parts(Glow plugs) are $20 each, is there any reason why we can't just install a $20 Block heater? I mean besides the question of plugging it in at work.



Just wondering



Pete

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Author: johnnyrisk Two stars, 250 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18318 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 8:13 PM
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Hello all,

I picked up the 300 Mercedes 300D Turbodiesel from the garage today. My mechanic said that the glow plugs are fine. This puts me back to the beginning. It's been warm this week, and the car started very quickly so I don't know if my starting technique was wrong or something else is amiss.

I want to thank everyone for their advice. I never expected to see this many repsonses to my post. When the temperature dips again, I will try to start the car per the instructions in the owners manual.

I did notice something yesterday when I was jogging in the neighborhood. One of my residents of my block has a big pickup truck emblazoned with "Andretti Racing Team" along with a racing trailer in his driveway. I noticed a long extension cord leading back to the house. On closer examination I saw the "V-8 Diesel" emblem on the front fender. The sly fox has a block heater on this very new truck.

If I continue to have trouble starting this car in cold weather, I will install a block heater. This will solve the cold start problem in the morning.

If you can't beat em, join em.

Regrards,
John Z.


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Author: vuelta Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18319 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 8:24 PM
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If I continue to have trouble starting this car in cold weather, I will install a block heater. This will solve the cold start problem in the morning.

Could this be a fuel problem? Doesn't diesel tend to gel or something at low temperatures?



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Author: peplow Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 18321 of 71152
Subject: Re: Any Diesel Heads Out There? Date: 2/21/2002 9:11 PM
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Could this be a fuel problem?.........................................


Could be, But if it's only a bit below freezing, I wouldn't expect TOO much trouble.
When was the last time the fuel filter(s) were changed? How BIG is the battery in this beast?

Of course, she's not a young girl any more :)





Pete

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