Cold Engine Knock
Okay so I'll get straight to the point. I have an 09 Cobalt with the LAP ecotec, 139,xxx miles. Bought the car with 122,xxx, no idea on previous service history, how it's been driven/taken care of. I was in desperate need of a car so I took what I could get. So far it's been pretty solid. I've had exhaust problems which I've fixed, had the dreaded fuel line leak, just fixed that a few weeks ago. But the engine has been fine. It's got plenty of power for a pretty much stock 2.2 (zzp downpipe). I replaced my spark plugs a few months ago and noticed I had carbon on my Pistons when I looked down into the hole. Possibly caused by cracked manifold? It might be running rich. That's not the problem though, just trying to give as much info as possible. For the past 2 weeks, whenever I start my car cold, the engine will produce this sharp, (but not loud like a rod would be) knock. It will start a few seconds after the car is started, and it does it at idle and most prominently at 2500 RPM. Finally it completely goes away after about 5 minutes of driving, until it's cooled down and started again. I've been reading about it a lot and well, I'm almost certain it's piston slap... I really don't want it to be. These ecotecs are such strong engines and my gf's ion has been run to shit at 190,xxx miles and doesn't make any sound like that, just the timing chain rattle at startup. I just want to see what you guys think. Any advice is appreciated.