What Am I Missing? Another Misfire Headache! - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 3 (permalink) Old 04-16-2017, 12:47 PM Thread Starter
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What Am I Missing? Another Misfire Headache!

2007 Cobalt 2.2

Engine was removed, machined, rebuilt, replaced. 4 new pistons, rings, crankshaft ground, refurbished w valve job, both timing sets replaced, all gaskets, plugs, etc. A full rebuild. When fired up first time on fresh build had a misfire on #2. I ignored it long enough to seat the rings and allow full temp warm up. shut it down changed oil and restarted and began searching for cause of misfire. After running down all the usual suspects, a leak down test revealed #2 exhaust side had a massive leak. This surprised me because the compression test showed 190 PSI like the other 3. I had assumed it would have shown somewhat differently than its siblings.

I inspected the valve lash adjusters and followers and found nothing suspect. In an attempt to cure without having to remove the head, I replaced the 2 adjusters and followers. Fail, no difference.

Removed head and found carbon behind all 8 exhaust valves. #2 was the worst by far and the culprit holding open was obvious. I determined the catalytic converter had failed and carbon and other crap was being sucked in to exhaust runners.

Cleaned all valves and seat areas. Vacuum test showed all ports (exhaust and intake) to be perfect. Replaced the catalytic converter. Re-installed cylinder head. Fired engine up. Runs far smoother but still a very faint misfire can be heard, no CIL though.

Connect the Tech 2 to learn #4 has a constant misfire.

Shut down and inspect plugs, injector, coil, and spray brake clean around intake and injector ports. No suspects. Swapped injector, plug, and coil from #1. Misfire still present on #2. Compression 190 PSI across all 4. Leak down test good.

Read this forum extensively and determined the next step should be a crankshaft (CASE) relearn. completed the procedure, no change.

Using the Tech 2, I observed the only misfire to on #4. The only hint of a change I noticed was that if I increased idle over 1800 RPM, the misfire log and graph stopped increasing. So thinking it meant the misfire was not present above idle, I decided to test drive. Fail. Definite miss present, down on power and flashing CIL for misfire.

Contacted my machine shop and they are scratching their heads too. They say sounds like a vacuum leak. That doesn't seem likely to me with these symptoms. I would expect a higher idle if that was the issue. Not many places a vacuum leak could hide on the 2.2. I disconnected the brake booster vacuum hose and blocked off the port on manifold. No change.

So, what am I missing? At this point no advice is bad! Headlight fluid, muffler bearings,
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post #2 of 3 (permalink) Old 04-20-2017, 12:59 PM Thread Starter
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Decided to swap sensors one at a time out of frustration and lack of better plan. No changes until I replaced the cam position sensor. The dead hole misfire moved to cylinder #1. I swapped it out with a used one and it moved to # 3.

I repeated the crankshaft learn (CASE) because I know the crank and cam sensor readings must match for computer to be happy. No change.

Decided to pop of valve cover and front cover and inspect timing. Inspection proved difficult due to marks not being close enough to painted links. So I just ripped it off and reinstalled it.

No change. #3 still misfiring.

Removed valve cover again and inspected cam and saw no build up on machined area where cam sensor gets signal from.

Removed intake cam anyhow. Replaced with another from an 07. Runs perfect. Inspect other cam again and learn it was from an 06. Machine shop swapped them out. They claimed they had several Ecotecs in for refurbishment. The 2 model years look identical, but do have different cast numbers.

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post #3 of 3 (permalink) Old 04-20-2017, 03:08 PM
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So they played a trick on you! Shame on them.... hell of a way to learn about an engine though.

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