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2010, Chevy Cobalt LS
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Discussion Starter · #1 · (Edited)
Hi all, I'm looking for advice on a continuous cyl 3 misfire issue.

I bought a used 2010 Chevy Cobalt LS (sedan, automatic) with about 92700 miles on it from a trusted local mechanic at auction. It drove fantastic for the first week (about 125 miles), and then began to misfire which quickly escalated. My code reader read P0303. It had an increasingly rough idle, would lurch violently after idling for too long, the check engine light would sometimes blink after these lurches, etc. It did not feel safe to drive.

Over the past month, I have:
  • replaced the spark plugs and swapped engine coils myself.
  • got a fuel flush, which helped, but the drive in service said it was still misfiring on 1st and 3rd.
  • got the 3rd spark plug replaced professionally (I didn't trust my work- turns out I cracked it). It no longer felt unsafe to drive, but continued to feel bumpy during idle. After this the car ran okay for about 80 miles then the light came back on with P0303. Brought it back, they suggested having the PCM reflashed at a chevy dealer.
  • Got the PCM reflashed at a dealer, didn't work.
  • Dealer ran a compression test. I was told cyl 3 is running at 50psi, and cyl 4 is running at 100psi, and that they couldn't diagnose the issue without taking the engine apart and I would need a new engine.
  • Called my seller (also a mechanic), he told me not to trust the chevy dealer, to change the 3rd cyl coil, and if the car was driving fine at higher accelerations don't worry about the rough idle and engine light (pretty frustrated with this!)
I'm at my wits end. This car was incredibly smooth to start, and I have tried very hard to be on top of this issue but appointment after appointment has failed to fix it, and now I'm being told I need a new engine for a car under 100k, which with my limited knowledge I know is ridiculous. My original seller, my current mechanic, and the Chevy dealer are all telling me different things are the problem, telling me to talk to the other, and at the same time telling me not to trust the others judgement.

Below I've transcribed what I was told by the mechanic, and what the dealer said. Both of these businesses have been around for years and have 5 star reviews. I'd appreciate any insight to what could cause this, what things can be done to help make it more driveable, etc. Is it worth pursuing replacing the PCM? Should I look into trying some things that might fix some compression issues without expensive diagnosis? Should I get another compression test with my mechanic? Say **** it, and accept my losses and live with a misfiring car? Some direction would be appreciated.

Thanks

Mechanic, after changing 3rd spark plug failed: Everything is functioning properly that would typically cause a misfire. Powertrain control module (pcm) seems to have a slight drop in voltage on the ignition side which is causing P0300 (random cylinder misfire) code to come on. Found cylinder 3 and 1 were slightly misfiring. Seems to happen when charging system is loaded and vehicle at idle. Recommend having dealer reflash the PCM, if that doesn't work. Would need to have PCM replaced and reflashed.

Chevy Dealer, after PCM reflash failed: 93112 attempted to reprogram ecm already has latest update. 93112 check for misfire on#3 cylinder. check for spark ok check for fuel injector pulse ok swap spark plugs between # 3 and 4cylinder no change. do a running compression test 50 psi on #3 cylinder 100 psi on #4. some type of mechanical trouble with #3 cylinder. quote used engine.
 

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If you’ve actually got such low compression on two cylinders, it’s hard to say what’s causing it. At lower miles I might suspect piston rings. Being that that seems to be something not so uncommon on the 2.2 engine. I had a Cobalt in my shop about a year and a half ago that had a similar issue and it was indeed needing a new engine. I highly doubt with what you’re telling me that it’s anything related to a bad PCM.

It really sounds like you may be needing to take it to someone else that knows what they’re doing and have them give their opinion on the issue. You will have to spend money to have someone else do a compression test but it would take the guessing out of your ordeal. If it was me I’d probably charge you a 1-1.5 hour fee to do the diagnostic work which is probably what a reasonable shop should charge to do a compression test and other tests like fuel pressure and checking things with a scanner.
 

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Discussion Starter · #3 ·
Thanks for your reply. Definitely not what I wanted to hear ahaha, but it's still good to have a second opinion! Knowing that these engines do indeed break sometimes at least cures my worry I did something wrong to cause this- silver linings.

I'm getting another compression test done. After the dealer I checked the codes and the P0303 was still there, but there was also a P0353, which is new. I'm thinking there might be some issues with the engine coil wiring with the new code? That could explain why this problem has fluctuated in severity. The car is actually running fine right now, and the check engine light turned off a few miles ago. I can feel some minor misfires while idling, but it is nothing like when the first week of this issue where the car became undrivable. Weird! Hoping my mechanic will have a better idea tomorrow.
 

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Discussion Starter · #4 ·
At lower miles I might suspect piston rings. Being that that seems to be something not so uncommon on the 2.2 engine.
Yep, that's the answer I got today. Fine compression (200-210) in cyls 1, 2 and 4. Cyl 3 had a dry test of 110 psi and wet test of 170psi. Can't afford to fix it right now but at least I can now find some things to help it run a bit better in the mean time.
 
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