Some back story: 09 Cobalt SS/TC. Its been laid up since last year, haven't had much time to work on it. Did a compression test initially (cant recall #'s but I knew 2 and 3 were still good and 1 and 4 were lacking) So I eventually pulled the head off found a handful of problems. Lapped the valves, didn't see any damage to them outside of the coating of crap around where they seal/seat. Didn't notice any head damage. had a lot of oil below the head, saw that the headgasket had basically taken a dump. Checked the injector seals for damage, Visually inspected the sleeve walls, didn't see any signs of scoring or cracks. At this point I should have gone further and removed the pistons and inspected the rings, rods etc, but after finding stuff like the coatings on the valves and the headgasket issue I figured I were in the clear and decided to reassemble. Car seemed good for a couple of hours idling, and then eventually sounded like she was falling on her face again. Tried to drive her around the block and would get some blue tinted smoke out the exhaust.

Waited a couple days intermittently starting it, while waiting for a leakdown tester that I ordered to come in. and this is what I found this morning doing a compression test first. Cylinders 1-4 in psi, 60,180,185,135. Did the leakdown test with instructions I found off a thread on here somewhere. And the loss/leakage %'s if I did my math right were as follows 73.5%, 3%, 2%, 35%. All the cylinders had some air coming out of the oil dipstick (As I know the rings aren't a perfect seal), but cylinders 1 and 4 sounded like they were losing air from the valve cover to the compressor side of the turbocharger, and after pressurizing the cylinders (one at a time of course) If i opened the oil fill cap you would hear the air abruptly leave those 2 in particular (cylinder 3 also did this but it was ever so slight in comparison). None of the cylinders showed signs of air leaking into the coolant or bubbling in the reservoir. I didn't hear or notice any air hissing out of either the intake manifold or the exhaust manifold. Should I have checked anywhere else to better determine the cause of the compression loss. Or should I at this point assume that its quite possibly my pisons/rings or something close to that vicinity. Or is there another place I should look while doing a leakdown test. Any information would be greatly appreciated.

 

The ringlands on 1 and 4 are probably cracked, you should take them out and take a look. That's what happened on my motor, similar symptoms.

 

Reading the OP, this is what I think too. If none of the valves were bent AND you just lapped them, they should be sealing pretty good. You probably have cracked ringlands.

You were at TDC on the cylinders when you were doing leakdown, correct?

 

Agree with the above

 

TDC on the compression stroke. Did each cylinder in their respective firing order 1-3-4-2, checked it multiple times to make sure I was at TDC on the compression stroke with similar #'s resulting. Going to pull the pistons at this point regardless, just debating whether I should just replace the ringlands if they are junk or if I should install new rods, pistons, etc. Thanks for the input =D

 

I'm gonna guess you won't get away with just replacing rings. The ringlands are part of the pistons and I'm guessing that's probably your issue.

 

Meant to say replace the rings* not the ringlands, but anyways. Thanks for all the input, it is much appreciated. Ill weigh out my options and proceed from there. Last question, go with stock pistons (lasted roughly 60k miles and I wasnt exactly gentle with her lol) or should I scope out what ZZP sells? Wiseco pistons etc...?

 

It really depends on what you want to do with the car. If you are gonna keep it for many years it's always best to go with forged pistons, which gm should have done from the beginning. Especially if you are gonna drive it hard. I would never put factory pistons back in unless I was going to drive like a grandma due to forced induction.