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Hey guys. My parents overheated their 2006 LaCrosse with series III 3800 engine. I first noticed that plastic overflow tank was full of coolant. So once the motor cooled down I removed radiator cap and an enormous amount of pressure was released and then the water from tank flowed into radiator. I then cranked motor with radiator cap off and water gushed out or radiator in buckets. I suspected a blown head gasket pumping back pressure into water system. Then I removed two of the front spark plugs to check compression and water pumped out of cylinders when cranked.

I started tearing down motor expecting a head gasket replacement job. I removed the Intake Manifold. But then I started reading from here and other places that it is a common problem was the IM gasket leaking on the plastic ones up to 2004. This would allow water to dump into cylinders and would most likely be my problem?

So...I decided to do a compression check with the IM removed, fuel pump disabled and belt removed. I assume I can do this and get accurate readings...right?
Given that I did the compression on front 3 cylinders I figured a head job was in order and didn't go through the hassle of pulling the back 3 plugs yet.
Anyway:
#1 - 60# psi
#3 - 75# psi
#5 - 140# psi

Does this verify that I at least need to pull heads and inspect and forget about thinking that I only have blown an intake manifold gasket? Just wanted to confirm before I started pulling the heads.

Few questions before I go further:
My UIM is aluminum. Is the IM gasket issue a common problem with these?
How would an UIM gasket cause pressure to pump water out of radiator? Is this normal? I would think head gasket?

Thanks
 

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Post is in a normal section.

I'd continue with the teardown, since it's highly possible an HG or both gave out. Overheating is never a good thing, and can extend problems from a simple place to a bad one.

UIM vs. LIM - the car being a Series 3 with aluminum upper intake has no influence on the issue with the lower intake manifold gasket issues that seal to the heads. This is basically a factory-admitted fix that all S2&3 3800's benefit from in the long-run (since they offered aluminum gaskets after the engine went out of production, if I recall the release year I looked up a few months back).

Cranking when water is possibly in the cylinders is very bad! This has the potential to hydro-lock the engine and/or bend a connecting rod, spin a bearing, etc. due to the water not being compressible like air when the pistons move up and down. Hopefully, you got away with one since the HG's might be letting excess pressure out for you right now. Pull plugs first and hand crank the motor to get any water out, next time, and drain coolant before doing a compression check.

Head gaskets and bolts are cheap to replace (maybe $100 total) and the whole job may run $200 or so for some gaskets, RTV, and fluids to get back rolling (played this game a few times as a poor college kid). Iron block and iron heads are pretty resilient here, so if you're able to give the heads a decent look, they may bolt right back on without machining (I've never sent heads out, and ran more cylinder pressure than most here).

Good luck on the work.
 
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