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Old 03-24-2014, 02:51 PM   #1 (permalink)
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09 Cobalt - Air temp related misfire

09 Cobalt XFE manual. I've had the car 2 years and think it's done it the whole time. But realized it went away this winter, back first day air temps broke 50f it did it again on the way home. Temp colder it went away.

Symptom: driving along light load 1500-2000 rpm, it stumbles for a fraction of a second, comes back on then repeats, sometimes just one, other time 5-8 times. No CEL. no codes via Scan guage. About 85,000 miles now.

It's more than a single misfire it almost feels like you hit a dip in the road as it's enough to slow the car. It's a drive by wire car.

When it happens in 3-4 gear 35 mph it really chugs, at 55 mph its just a dip by comparison. 90%+ of my driving is 55 mph.

I have doubt is cutting ignition or fuel alone would chug like it does, it's almost like the throttle is slamming shut and reopening. The SG2 display doesn't seem fast enough to catch what's going on.

Suggestions?

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Old 03-24-2014, 03:00 PM   #2 (permalink)
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So the best guess I can give is pre-ignition from the higher air temps. I assume you are using a wai?

Does that car use VVT?

Best software I have had experience with is OBD2007 for data logging. I can only suggest cleaning the throttle body and appropriate sensors/air control valves, as well as check/replace spark plugs. Maybe use a higher octane and observe changes.
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Old 03-24-2014, 04:45 PM   #3 (permalink)
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All stock motor w/VVT. SG2 isn't plugged in at the moment, intake air during winter was only a couple degrees above ambient. Been running lots of different fuel blends, going with E10 87 octane for a while till I figure it out. Some days I think it's not going to make it, will do it 30 times, then gone for weeks. Didn't pick up the air temp correlation till few weeks ago, back in to 20's today and it's OK. Will see on way home it will probably do it now.

I'll was thinking data logging will be the way to go but no experience with it, will look into that.
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Old 03-24-2014, 04:54 PM   #4 (permalink)
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data logging will be your friend, possibly a dying sensor.

I had an issue with a egr valve that was slow to close causing the engine to die when you let off the throttle, only way i found the problem was seeing log data of the IAT sensor reading a 100 degree change in under 10 seconds.
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Old 03-24-2014, 05:58 PM   #5 (permalink)
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Could also be O2 sensor or another critical sensor. You can tell if you unplug your O2 sensor. The car will run in open-loop mode, ignoring sensors like the O2 sensor. If the problem goes away, you can rule out things like plugs, ignition coils, fuel supply, etc. and focus on closed-loop components for troubleshooting.
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Old 03-27-2014, 03:04 PM   #6 (permalink)
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38 & wet this morning and all fine. I keyed off and back on to see if it would feel the same and it does, just quicker than I can do it with the key.
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Old 04-28-2014, 11:28 PM   #7 (permalink)
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Missfire is getting worst, watching the SG2 on way home, GPH went to 0 or real close at times even wasn't missing. TPS doesn't seem to change much from 25 at idle with limited sample of data seems like 35-38 on tps is where it cuts out. I'm leaning towards MAF, only if my other cobalt was closer I could swap and test. Did a couple 2nd gear to 6000 rpm pulls and it no misses, also did 5th gear up hill 1300 rpm to 2000 (55 mph) again no misfires.
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Old 04-29-2014, 10:23 AM   #8 (permalink)
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Team Chevy
90 day: 39.31 mpg (US)

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Finally got a code PO102 indicating MAF this morning. After it came on the car ran fine, it was still running closed loop wonder if the car's ignoring the signal it's receiving. Did see GPH go to 0 a couple times afterwards but not as much as before but without the misfire I wasn't looking at the SG2.
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Old 04-29-2014, 02:28 PM   #9 (permalink)
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Impala has the same part, going to swap MAF with it tonight and see if that fixes it.
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Old 05-08-2014, 10:33 AM   #10 (permalink)
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Didn't get around to swapping parts. Got a can a MAF sensor cleaner instead and used it over the weekend, I think it missed once since cleaning.

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