Missfire P0300 - Chevy Cobalt SS Forum : Chevrolet Cobalt SS Forums
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post #1 of 3 (permalink) Old 05-01-2010, 11:54 AM Thread Starter
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Missfire P0300

Hey guys,

Chev Cobalt SS 2006
59000 miles

so here is the problem, while driving on the interstate the car started having a serious misfire. we pulled in a autozone in order to get the codes pulled from the computer since the SES light was blinking.

The codes were a p0303 thats indicated a misfire in cylinder no 3.

We looked on the web and found a TSB that indicated that these models can have a harness rub and short to ground, however we preformed the procedure and no lines were damaged and shorted to ground.

We pulled the plugs and found our first clue at the problem.

As you can see the plug in no 3 cylinder was completely shot.

We replaced the plugs with champion 9001 iridiums. The plugs were shorter that the gm plugs witch is weird, but the clerch at the store assured us it was the correct plug for the model.

after replaceing the pugs and the no 3 coilpack the problem still persist. in fact we checked the sparks and all 4 coil provide spark to the plugs.

When we looked with a flashlight we could see that there was raw fuel in the cylinder no 3.

So what could be causeing a no combustion situation at this point?

Last edited by Alexduty; 05-01-2010 at 12:26 PM. Reason: additional details
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post #2 of 3 (permalink) Old 05-01-2010, 06:52 PM
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Posts: 1,329
lets hope that the missing piece of the plug didn't cause piston damage.. sorta sounds like it did...


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post #3 of 3 (permalink) Old 05-01-2010, 07:20 PM
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Well I would think the plug would have burned up slowly until it reached it's point now. What I think a problem could be that the plug didn't fire for so long that gas just dumps in and just eats the oil barrier around the piston doing damage to the cylinder wall. Hopefully not but I know some cars can stop injector pulse to the misfiring cylinder. What I would do is if it just miss fires sometimes take it on the high way at a cruising speed and just let the engine get more oil to that spot

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