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Old 03-16-2009, 09:03 AM   #1
mrpromax
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Question '07 Cobalt Bad engine misfire

First I would like to that everyone in advance. I am new to this forum and just need a little advice. Have an '07 Cobalt 2.2 with 28k miles. At about 27k miles it started to miss a little at idle when it got up to operating temp. Then just recently the miss became very bad and finally got a P0300 code which is engine misfire. I checked the spark plugs and they are white indicating that the motor is running lean. A friend told me there is a TSB that states you have to change all the injectors and reprogram the computer. The vehicle is not under warranty because it is a rebuilt title. (Roof damage from a tree falling on it) so nothing that would affect the motor.

Any ideas other than that TSB ? I just want to see some opinions before I go through the expenditure.

Thanks in advance,
Eric
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Old 03-16-2009, 01:14 PM   #2
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Re: '07 Cobalt Bad engine misfire

Are you positive there's no water in the fuel tank? I've seen random misfires (P0300) caused by that. A vacuum leak will also cause a 300, as well as a MAF or MAP sensor issue. It's possible all 4 fuel injectors are fouled beyond repair also.

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Old 03-16-2009, 01:18 PM   #3
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Re: '07 Cobalt Bad engine misfire

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Originally Posted by rodeo02 View Post
Are you positive there's no water in the fuel tank? I've seen random misfires (P0300) caused by that. A vacuum leak will also cause a 300, as well as a MAF or MAP sensor issue. It's possible all 4 fuel injectors are fouled beyond repair also.


Joel
Thanks for the response. How can I tell if there is water in there ? I've gone through maybe 20 tank fills since it had the problem. Should I add a water remover to the tank and clear the code to see what happens ?
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Old 03-16-2009, 05:51 PM   #4
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Re: '07 Cobalt Bad engine misfire

It wouldn't hurt, but like you say, with that many fillups, it's not likely your problem. If it is water, you'll notice wet-ish sparkplugs and they won't smell strongly of gasoline.
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Old 03-17-2009, 11:21 AM   #5
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Re: '07 Cobalt Bad engine misfire

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It wouldn't hurt, but like you say, with that many fillups, it's not likely your problem. If it is water, you'll notice wet-ish sparkplugs and they won't smell strongly of gasoline.
The plugs are dry and the electrode and end are white. Lean for sure.
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Old 03-17-2009, 01:06 PM   #6
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Re: '07 Cobalt Bad engine misfire

Does it start up and run perfectly fine when cold?
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Old 03-17-2009, 02:44 PM   #7
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Re: '07 Cobalt Bad engine misfire

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Does it start up and run perfectly fine when cold?
Yes it does. Everything is fine when it's cold.
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Old 03-17-2009, 03:01 PM   #8
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Re: '07 Cobalt Bad engine misfire

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Yes it does. Everything is fine when it's cold.
It sounds to me that once the ECU jumps from open to closed loop mode and needs to read off all your sensors (O2, MAF, IAT, etc...), something is causing the fuel trim to fall too lean. You'll only go into closed loop when the engine is up to temp, not near full throttle or under heavy load. R U sure you don't have any other codes?

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Old 03-17-2009, 03:10 PM   #9
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Re: '07 Cobalt Bad engine misfire

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Originally Posted by rodeo02 View Post
It sounds to me that once the ECU jumps from open to closed loop mode and needs to read off all your sensors (O2, MAF, IAT, etc...), something is causing the fuel trim to fall too lean. You'll only go into closed loop when the engine is up to temp, not near full throttle or under heavy load. R U sure you don't have any other codes?

Joel

Yep. Just reran the scanner to make sure and I'm getting just the one code.

After that, I started the motor and it runs great. Wierd huh ?
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Old 03-17-2009, 05:24 PM   #10
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Re: '07 Cobalt Bad engine misfire

If you let the scanner go to the point it reset the code, the ECM has now been cleared of all it's 'learned' variables. You'll run in open loop mode now for something like 50miles or a bunch of start- warm up - stop cycles. Not a bad thing, you just don't want to do this and then take the vehicle in for a safety/smog inspection. It won't pass while in open loop mode. Honestly though, I'd pop the top half of the aircleaner housing off, unplug the connection to the MAF, spray it out really good with spray MAF cleaner or spray electronics cleaner and see if the issue stops.

Joel
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Old 03-22-2009, 02:07 PM   #11
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Re: '07 Cobalt Bad engine misfire

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Originally Posted by rodeo02 View Post
If you let the scanner go to the point it reset the code, the ECM has now been cleared of all it's 'learned' variables. You'll run in open loop mode now for something like 50miles or a bunch of start- warm up - stop cycles. Not a bad thing, you just don't want to do this and then take the vehicle in for a safety/smog inspection. It won't pass while in open loop mode. Honestly though, I'd pop the top half of the aircleaner housing off, unplug the connection to the MAF, spray it out really good with spray MAF cleaner or spray electronics cleaner and see if the issue stops.

Joel
Tried cleaning the MAF, adding a water additive, and a fuel injector cleaning. Nothing. Same thing. But again, it only happens when the motor gets warm. I think you are on to something with the open and closed loop computer thing but I can't pinpoint what sensor is making the car run lean. Not getting any other codes.
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Old 03-22-2009, 05:21 PM   #12
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Re: '07 Cobalt Bad engine misfire

Man that is frustrating! Still just the P0300 / random misfire code? Is it possible for you could get ahold of a more advanced scanner that will give you pending and/or manufacturer specific codes? The <$300 readers just give you generic OBDII codes. Only other thing I could think of is swap MAFs with another ~2007 Ecotec 2.2 to see if the problem goes away. *ahem*.. maybe rent a 2008.. (wink-wink.. nudge..nudge). The MAF could be bad. O2 sensors don't jerk the fuel trim around enough to cause misfires, so I'd rule them out. Also can't see all 4 coil-overs being bad to cause random misfires.

Joel
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Old 03-23-2009, 06:51 AM   #13
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Re: '07 Cobalt Bad engine misfire

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Originally Posted by rodeo02 View Post
Man that is frustrating! Still just the P0300 / random misfire code? Is it possible for you could get ahold of a more advanced scanner that will give you pending and/or manufacturer specific codes? The <$300 readers just give you generic OBDII codes. Only other thing I could think of is swap MAFs with another ~2007 Ecotec 2.2 to see if the problem goes away. *ahem*.. maybe rent a 2008.. (wink-wink.. nudge..nudge). The MAF could be bad. O2 sensors don't jerk the fuel trim around enough to cause misfires, so I'd rule them out. Also can't see all 4 coil-overs being bad to cause random misfires.


Joel
Let me see what I can do to get someone to loan me a better scanner or just pay someone for the diagnostic. I'm with you on the coil comment because the spark plugs are white. It's obviously fuel related and not the coils. What do you think about the TSB that states that you have to replace all 4 injectors & reprogram the computer ?
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Old 03-23-2009, 02:28 PM   #14
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Re: '07 Cobalt Bad engine misfire

I don't have a lot of faith in the replace all four injectors deal. The only thing that would ruin all 4 simultaneously would be some serious contamination in the fuel (plugged/clogged/dirty injectors). Other than that, like the coils, all 4 wouldn't go bad at the same time. Low pressure at the fuel rail could cause a lean condition, but why it only happens when warm is a mystery to me.

Joel
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Old 03-25-2009, 05:37 PM   #15
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Re: '07 Cobalt Bad engine misfire

Can your scanner read some data such as operating temperature and O2 sensor switching. You really need a scan of various data to pin point what is going on.
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