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Discussion Starter #1 (Edited)
Hey folks
New here and have searched a number of P0303 misfire threads but never really saw anything with a resolved repair.
Cobalt is 2007
I'm getting the code P0303 misfire cyl 3. At first it was random now it seems to be permanent. I also ran Live Data on my scan tool it its reading Fuel System 1 Open Loop Fault only at idle. This kind of leads me to the H02 sensor but theres no code stored for that in the computer.

I've done the normal maintenance stuff
plugs
air filter
oil change
swapped coil pack 1 to 3
fuse check

Any and all advice will be appreciated.

thanks
 

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Discussion Starter #3
Injector or bent valves. Does the engine shake on you or cut out? Try swapping injectors

Sent from AutoGuide.com Free App
I don't think its a bent valve. If it was it would be constant.
I just got in from more troubleshooting and found the TB was pretty dirty so I cleaned that up and it ran great. I though I had it. Nice smooth idle, steady at about 910rpm and only a P0102 pending code after clearing the codes, since I had it running with the intake off. I shut it down, and reinstalled the intake, and it progressed down hill from there until the idle dropped to 655rpm and it started shaking, logging the p0303, and my open loop fault was back.

I then removed the intake boot shroud while running and it eventually, not instantly, smoothed out, cleared the open loop fault, and the idle ran back up to 910rpm.

I'm starting to think the MAF is in play now.
 

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P0303 is only valid if it is accompanied by a double digit amount of misfire error counts.
 

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Discussion Starter #6
Updates

I've since replace the MAF and that did nothing to resolve this issue.
I also removed the intake and checked for restriction, and it was clear.

I also noticed on my live data scan I have Bank 1 Sensor 2 reading .965v STFT 99.0%.
Were Bank 1 sensor 1 is reading STFT 0.0% at idle.

I don't know what those are an indication of, but I have no o2 sensor codes.
Can anyone help?

thanks
 

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Those readings look about right. You kinda need a scope for the first one because it's going to be all over the place. Second HO2 is more stable at just under a volt.

---------- Post added at 08:28 PM ---------- Previous post was at 08:19 PM ----------

How many misfire counts do you have for cylinder 3?
 

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Discussion Starter #8
P0303 misfire resolved!

I think I can finally put this problem in the past.

Running a live scan on my OBD II scanner indicated a open loop fault condition when idling at running temp.
This should have been an alert to a faulty o2 sensor.
I bought both the up stream and down stream o2 sensors, and received 2 up streams. So I went ahead and replaced the up stream. And I'll replace the other one soon.
After 30min driveway run time, the problem is gone. No codes and no open loop fault.
 

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Well, that's a bit funny because if it was an HO² sensor you would think you'd get an HO² code. The ECM checks for 6 different possible fault with each HO²s. One possibility though is that the HO² in question might be out of specs enough to make your engine run like crap enough to set a P0303 but not enough to set an HO² fault code.
 

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Discussion Starter #10 (Edited)
Well, that's a bit funny because if it was an HO² sensor you would think you'd get an HO² code. The ECM checks for 6 different possible fault with each HO²s. One possibility though is that the HO² in question might be out of specs enough to make your engine run like grap enough to set a P0303 but not enough to set an HO² fault code.
I agree and I'm a little pissed since I replaced MAF ($35), Injectors ($25), and Plugs($24), + countless hours, all unnecessarily. At least the parts were cheap...
Those same parts for my motorcycle would have been 2x the cost each.

One interesting note, after replacing the up stream 02 I am now seeing much different voltage readings on both o2's running live data on my scan tool.

Hopefully this info is of some help to others in the future. I have seen posts of dealerships service shops indicating new heads or complete engine rebuilds were required to resolve P0303. I was 100% sure that was a load of bullsh*t.
 

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Discussion Starter #11
Recently the problem returned.

Ran a compression check and found cyl 3 reading 70-80 lbs. The other 3 cyls reading 180-190.

So I guess I have a compression problem. How does the OBDII know this?
 

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Thanks for confirming this to us. False P0303's mystery is slowly being decoded.
 

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My problem is in cyl #2......low compression......at idle she runs like crap at anything over 2k rpm she runs great....and I still get 28+ mpg....and @180k on her, I dont want to rering.......she dosnt blow smoke.....so lm not sure what the true issue is.
 

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You might have debris like carbon stuck in one of the valves. Maybe seafoam to clean it.
 

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No need to pull the head. They now have really small cameras you can insert through the spark plug hole. The head pivotal so you can see clearly everything in there. Even has it's on light. I believe it is made or distributed by snap on.
 

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Discussion Starter #18
Well my problem sort of went away.
My daughter totaled the car running into the back of an SUV on an interstate off ramp.

I had come || close to pulling the trigger buying a head on line the day before she totaled it. I was just waiting for a return email with casting number confirmation from the place I was buying it from. Glad the guy took his time about it.

I have to say what a relief it was to be done with this car. I though the Cobalt was evolution of the Cavalier which I always though was a solid vehicle. I tossed $7k out the window with this POS since we gambled with no collision coverage, and I'm still happy to be done with it. I had also put another $2k into maintenance in the 2 yrs we owned it.
I would not be able to sleep at night if I had sold this car to someone.
Regardless of the running condition at the time.
haha
 

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Daughter Okay?
 
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