Serious Misfire, No CEL, No Codes. Any Ideas? - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 8 (permalink) Old 09-04-2010, 05:58 AM Thread Starter
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Thumbs down Serious Misfire, No CEL, No Codes. Any Ideas?

I just bought my 2009 Cobalt with only 5k miles on it. It's an LS 2.2 XFE, manual trans. It stutters at times when accelerating in any gear, and when cruising at constant RPM's in any gear, it bucks like it has a bad miss, or it's not getting fuel or spark intermittently. It has yet to throw a code. I have had it at the dealer twice now and they say they can't duplicate. I checked the MAF sensor and cleaned it with the proper cleaner even though it really didn't appear to need it and got no results. I also treated the fuel with Heet just in case I somehow got bad gasoline. Still nothing. Anyone have any suggestions as to what the problem can be? I am begging for CEL at this point...
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post #2 of 8 (permalink) Old 09-05-2010, 02:06 PM
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I get the same thing every so often, a couple times it was just bad gas and another time it was my GMPP intake, the seal came loose after the MAF sensor

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post #3 of 8 (permalink) Old 09-05-2010, 02:09 PM
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could be the throttle body. won't throw a code unless it's getting a voltage correlation issue. out of calibration maybe.

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post #4 of 8 (permalink) Old 09-05-2010, 03:26 PM Thread Starter
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Thanks for your replies, I was beginning to think no one would.

Here is what I have found so far... I disconnected the PCM for an hour or so, so that I could reference the number on the back to see if somehow the previous owner had it swapped for whatever reason. I am convinced that this is an electrical/issue and not a mechanical one. The Julian code indicates that it is likely original to the car so I reinstalled it. While under the hood I went ahead and removed the intake from the throttle body to check for any vac leaks or anything out of the ordinary. I decided that for sh*ts and giggles, I would drive it a few miles to see if anything changed having the MAF sensor disconnected and the throttle body open to breathe freely. Guess what... it drove so smooth. All the bucking and stuttering completely went away. Of course I got a CEL, finally, indicating that the MAF sensor has low voltage since it was disconnected, but at least I know my CEL is functioning and wasn't tampered with. So then I reinstalled the intake and plugged in the MAF sensor. I have driven it on about 4 separate occasions since putting everything back together and the car is running perfect. The CEL even went away now that the MAF sensor is recognized. This tells me that the PCM is likely to blame for the hiccups the car had before. The question is, is this going to be a long term fix, or will it happen again? I am curious if by unplugging the PCM reset it somehow. What do you guys/ladies think?
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post #5 of 8 (permalink) Old 09-05-2010, 07:26 PM
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5000 miles?... dealer time!

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post #6 of 8 (permalink) Old 09-05-2010, 07:48 PM
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Thanks for your replies, I was beginning to think no one would.

Here is what I have found so far... I disconnected the PCM for an hour or so, so that I could reference the number on the back to see if somehow the previous owner had it swapped for whatever reason. I am convinced that this is an electrical/issue and not a mechanical one. The Julian code indicates that it is likely original to the car so I reinstalled it. While under the hood I went ahead and removed the intake from the throttle body to check for any vac leaks or anything out of the ordinary. I decided that for sh*ts and giggles, I would drive it a few miles to see if anything changed having the MAF sensor disconnected and the throttle body open to breathe freely. Guess what... it drove so smooth. All the bucking and stuttering completely went away. Of course I got a CEL, finally, indicating that the MAF sensor has low voltage since it was disconnected, but at least I know my CEL is functioning and wasn't tampered with. So then I reinstalled the intake and plugged in the MAF sensor. I have driven it on about 4 separate occasions since putting everything back together and the car is running perfect. The CEL even went away now that the MAF sensor is recognized. This tells me that the PCM is likely to blame for the hiccups the car had before. The question is, is this going to be a long term fix, or will it happen again? I am curious if by unplugging the PCM reset it somehow. What do you guys/ladies think?
In the future do an erase feature on your ECM instead of unplugging the battery. Doing an erase function resets all the sensor data back to the default and the ECM starts "learning" again. Unplugging the battery only does this if the internal ECM power supply is bad/low. I suspect your MAF may be the culprit -- you took it "out of the loop" and caused everything to reset. If your problem comes back you might want to "push" the dealer to replace it. Sometimes you just can't get the MAF to show a problem.

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post #7 of 8 (permalink) Old 09-06-2010, 05:56 AM Thread Starter
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In the future do an erase feature on your ECM instead of unplugging the battery. Doing an erase function resets all the sensor data back to the default and the ECM starts "learning" again. Unplugging the battery only does this if the internal ECM power supply is bad/low. I suspect your MAF may be the culprit -- you took it "out of the loop" and caused everything to reset. If your problem comes back you might want to "push" the dealer to replace it. Sometimes you just can't get the MAF to show a problem.
The battery disconnect was done prior to disconnecting and removing the PCM. It wasn't until after I disconnected and reconnected the PCM that all my problems went away, so I am assuming that by doing so, I erased the data in the PCM causing it to relearn from scratch. Is there another better way of doing this PCM reset in the future without having to take it to the dealer and put a tech 2 on it? It appears as though this has resolved the issues for now, but I agree that the MAF sensor may just need a little more time to send some more bad data to the PCM. Either way I think it is safe to say that the MAF sensor or the PCM itself are behind the driveability problems I was experiencing. It is just hard to narrow it down when no codes are being thrown.

---------- Post added at 08:56 AM ---------- Previous post was at 08:52 AM ----------

Quote:
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5000 miles?... dealer time!

423
I agree, in a perfect world, you should be able to take the car to the dealer and it come out running perfect as it should. But when the dealer doesn't spend much time diagnosing the problem and tells you more than once that they can't duplicate the problem, you have to start playing with the car yourself. Glad I did too because it seems like I was able to narrow it down to two possibilities and for the time being the car is running awesome.
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post #8 of 8 (permalink) Old 09-07-2010, 11:36 PM
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Quote:
Originally Posted by 423 View Post
5000 miles?... dealer time!

423
Agreed, the code sits in your ECM for, i think, 30 cycles of the key, then it gets erased. So as long as the light is still on and the code isn't erased yet, you can get the dealer to scan it and find out the issue

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