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The vehicle is a 2002 GMC Sonoma 4.3L Vortec.
I have had minor coolant loss for about 3 years.
A couple weeks ago the engine overheated (almost redline but not quite - about 250 degrees). The coolant reservoir was empty and I couldn't see anything in the radiator cap. I put about 3 quarts of coolant in the radiator and a quart in the reservoir and drove it to a mechanic. It didn't overheat at all on this trip of a few miles. When I got to the shop I left it running, got out and checked for dripping coolant anywhere and didn't see any, turned it off and still didn't see any, checked the reservoir and the level hadn't changed. The shop checked it out, pressure tested the radiator and coolant was guzzling out from somewhere. They did a lot of looking with mirrors and determined it was coming out of the weep hole of the water pump. They replaced the water pump, flushed the system, filled it, I was on my way, and for the next few days everything was normal, temperature was good and coolant levels were good. Don't know if it's relevant, but the water pump and the lower intake manifold gasket are very close, and they had a hard time getting an eye on the source of the leak - but since the new water pump seemed to atleast temporarily fix the leak I don't know. Why the system held the coolant on the trip to the mechanic - I have no clue.
Then, 4 or 5 days later, I was driving at about 45 mph and suddenly there was a power drop from the engine, it seemed to be firing sporadically, and it quit. Coolant was gone and temperature was about 240, towed it to the nearest mechanic, he said blown head gasket without any diagnostic testing (compression test or block test) other than what I told him. He tried to start it, and it seemed to try to turn over and almost start but wouldn't stay running. (Wet plugs?) Oil dipstick seemed to show that all the coolant was in the engine. Due to financial problems I just had it towed home so I could figure out what to do from there. I have a back up truck so in spare time decided to try and go at it myself. I went off his diagnosis and decided to try and go at it myself, no engine experience but lots of other random repair projects. I didn't know about the compression test or block test, nor did I know about the GM Dexcool issues and the common intake manifold gasket problem caused by the Dexcool. So here I am now, I have everything apart with the intake manifold off and the gasket is shot to hell. So now my question is, since I know now that the lower intake manifold gasket is bad, how likely is it that the head gasket is blown? Could the intake gasket have led to the head gasket? I've had the truck for about 4 years and it has never redlined since I've had it and a couple of times prior to all this it got up to probably 245 degrees a couple times. Based on those temperatures of overheating how likely is it that I have warped heads? Cracked heads would have been obvious and unlikely right? Based on the suddenness of how it happened is it likely that it was the lower intake manifold gasket that gave way suddenly? Or would that be the bit of info that proves it's the head gasket? Could the lower intake manifold gasket have had a small leak, hence the minor coolant loss, and then suddenly gave way the rest of the way? Also, I did not see smoke from the tail pipe and I feel as though I would have noticed it because even though I don't specifically remember doing it, I'm pretty sure I would have been looking in the mirror while getting over to the side of the road and seen it if it was there.
How the radiator system works - when it pressurizes and circulates/engages or whatever - I just have no clue, but it all seems relevant. I know now that I should have done much more research so that I could better try to diagnose it prior to taking stuff apart, but this is where I am. I don't know anything about the compression test, info on that would help. Now that I see the intake manifold gasket is bad I really hope this is the problem and I can put her back together again from here. It seems as though if I learned more about which ports within the intake go to where may help to decide but I don't know. The main question remains though - head gasket or not? How likely is it really that it's both? Any help would be greatly appreciated! I can post pics with manifold removed if needed.
Thanks - Tom
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