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[2006 Chevy SUV, 3.4L V6]

After I got home last night from a short drive and shut down, I heard a fan come on. It was a very warm evening, so I dismissed it as the fan doing its job and put off till this morning to check it. A few seconds later it stopped. This morning I popped the hood and checked the coolant reservoir level: dry as the Mohave. Looking around, I found some caked over rusty connections on the other (right) side of the engine, they appeared old and not active leaks.

Reservoir: https://ibb.co/w675j97 other end of engine https://ibb.co/887NqZj

I refilled the reservoir with fresh 50/50 coolant, and (right or wrong) leaving the cap off it started up. The coolant started spitting out and within a couple of minutes the reservoir was empty again. I refilled it again, up to the "cold" level, but I thought I'd ask as to what to expect before
starting up again, this time with the cap tightened.

In 50 years of driving I never had a radiator/coolant issue, so it's new territory to me. Please advise.

RayB
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What you pictured was the overflow tank. Is that the cap you left off? Or was it the actual radiator cap?

It is not very difficult or expensive to pressure check a radiator. Many leaks could be internal so you would never see. You can do a HC test on the coolant to see if there are any internal leaks are allowing hydrocarbons or combustion gasses to mix with the coolant.
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If you are getting bubbles/spitting through the coolant overflow reservoir, and losing coolant out through its vent, even when the engine is cold, it might be a sign of a failed head gasket. This happens if combustion gasses find their way from the cylinder, past the head gasket, into a coolant passage in the block.

Hope I'm wrong.

xtn
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At some point, if indeed a head gasket, it may become water blocked, can't compress water, but I'd expect to see a lot of exhausted water, steam before then... Sparkplugs don't work well when wet, ether, so have missing, too. Needs a real workup, diagnosis..
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Would have to be pretty bad for water to get into the cylinder. If it's minor, combustion pressure can push a little over into water jacket, but water pump pressure cannot push water the other direction. But yeah, it will only get worse. I would do a compression test on the cylinders, with the radiator cap off, to see if any leakage gets through.

xtn
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I would do a compression test on the cylinders, with the radiator cap off, to see if any leakage gets through.

It can work the other way, too. If one cylinder shows significantly higher compression than the rest, it could be because there is coolant in the cylinder.

Been there, done that.

Neil
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