Compression test #3 cylinder

[rahzip]

08 GMC 5.3 65000 miles

Previously this engine had some collapse lifters. A GM dealer made the repairs.

Now, the engine has a miss. The P0300 code comes up along with service stabiltrac. Cylinder #3 seems to be the culprit. I have checked the compression. When I check it and just crank the engine (engine does not start) I get 130psi. If the engine starts I get 0psi. If I wait a while and crank it (no start) 130psi. Valve springs are OK.

Has anyone else seen this?

[Doug_Scott]

First thing to do is a proper compression test. Remove all spark plugs, remove fuel pump fuse (maybe labeled INJ 1), remove fuse for ignition system, or unplug coil packs, and hold throttle wide open for the compression test. You need to check all 8 cylinders, and then compare the results to all cylinders. The results should be all about the same across all cylinders. Not sure what the current guides say for acceptable difference, but it used to be 15%. Compression should also be above 100psi.

If you have a low cylinder, squirt some oil into it, and check compression again. A big swing in compression after adding a bit of oil should lead you towards an issue with rings on that cylinder. If there is anything wrong with a valve, it will generally show up as a big difference in compression, with almost no difference when testing wet(after oil is added).