Need Major Help with a Warm Idle Misfire!!! - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 14 (permalink) Old 11-15-2012, 11:45 AM Thread Starter
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Exclamation Need Major Help with a Warm Idle Misfire!!!

I bought a used 05 cobalt 2.0 sc around 3 months ago, from the beginning I've constantly been replacing parts to get the car up to par. My current battle is agents normal operating temp range misfires and slight engine vibration. You can hear the miss from the exhaust and I've about fully extended my wallet. The cars bone stock except for a typhoon cold air intake. Parts I've replaced would be maf, upstream o2, camshaft position, cleaned throttle body, new head gasket and intake, new timing chain, fuel filter, new plugs gap .32 nkg, new air filter, cleaned grounds, new battery, I've about given the car the whole once over. The only thing I've been able to do that seemed to even it out was to disconnect the pcv hose from the valve cover which connects to the cai and suck air from it. In my opinion I find that quite odd but I'm in uncharted territory for my knowledge. Any help would be greatly appreciated, I also have a snap on diagnostic scanner so I can provide sensor data, I'm just unable to Interpet it.
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post #2 of 14 (permalink) Old 11-15-2012, 12:33 PM
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post #3 of 14 (permalink) Old 11-15-2012, 01:05 PM Thread Starter
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So far I've only had a p0300, it causes a check engine every once and a while, as for when it's off I can watch the missfire data, once it warms up cylinder 2,3,4 go to missing. I've changed the coil packs and injectors around with no change in the misfire pattern. I also did a compression check left to right it's 150,145,125,150.

---------- Post added at 01:05 PM ---------- Previous post was at 01:03 PM ----------

Also once around 1,000 rpm and up it stops missing?
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post #4 of 14 (permalink) Old 11-15-2012, 02:09 PM
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Almost sounds like its way to rich. That would explain the unmetered air helping it. Have you watched your a/f ratios?

Did you get the head checked over when you did the gasket? Also I've seen coolant temp sensors act up and not set off a code. They can make a vehicle flood itself

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post #5 of 14 (permalink) Old 11-15-2012, 02:17 PM Thread Starter
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No stupidly I didn't have the head checked, but on the good side I haven't lost any oil or coolant. It's not soo much unmetered air as it is taking metered air away if that makes sense lol afr seems to stay at 14.7, I also noticed my lower bank o2 sensor doesn't fluctuate really, it stays about 788
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post #6 of 14 (permalink) Old 11-15-2012, 02:26 PM
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The rear o2 should move. It might be getting bad. My car lost 7mpg when it failed
If you're pulling a hose off the intake and it smooths out its usually because the vacuum in the intake sucks in the air it wants.

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post #7 of 14 (permalink) Old 11-15-2012, 02:31 PM Thread Starter
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It's the hose the runs to the back of the valve cover that acts like a pcv, when I unplug it from the valve cover there is no change, but with the hose still connected to my cold air intake piping If I suck a curtain amount of air from the cai tube the motor evens out.
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post #8 of 14 (permalink) Old 11-15-2012, 02:35 PM
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Is there a pcv valve there? I'm not super familiar with the supercharged cars

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post #9 of 14 (permalink) Old 11-15-2012, 02:42 PM Thread Starter
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No it's just a tube that goes from the air intake piping to the valve cover, the air moves through passages in the head and block, that's all I know haha. Oddly enough I can have the hose hooked up to the valve cover, unhook it from the cai "without the car running" and hear an air sound when I blow air into the crankcase? I pulled the fuel injectors out and did it again, the air I blew into the valvecover seems to be traveling into the intake. If that's normal I haven't a clue lol
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post #10 of 14 (permalink) Old 11-16-2012, 05:19 PM
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I agree with the temp sensor comment,maybe even a semi clogged converter? Check exhaust temp with your scanner and see how hot its getting.

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05 cobalt ss , lsj , missfire , p0300 , vibration

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