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post #1 of 3 (permalink) Old 08-16-2016, 09:33 AM Thread Starter
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Endless Misfires in My '08 2.4l LE5 Cobalt

Hi everybody!

This has been an ongoing issue. I have a 2008 Cobalt Sport and am constantly getting misfires. Occasionally, it will trigger the P0300, but most of the times it just constantly misfires @ idle and WOT!

I've replaced all the spark plugs (4 times), all of the coils, the MAF, Ive checked for vaccuum leaks more times than you can count on the intake manifold, the throttle body, the maf, any vaccuum lines; ive replaced the fuel filter, checked the fuel pressure, and have changed the injectors! The engine in it now isnt the original. It got shipped from LKQ as the last engine i had the timing went off and a piston smacked the valves. But according to them they compression test it before it leaves. Im inclined to believe them as they dont want any warranty claims.

So, to say the least it has been straight HELL with this car. But for a while there i was gut feeling it that it was from messed up fuel injectors. Well, before this engine i bought a supercharger kit and was waiting to install it. Well it came with new injectors. So to justify it i thought ill just put the kit in since it has new bigger injectors.

To make a long story short, i got my computer flashed to be able to run the supercharger kit. Ive changed the gaps on the plugs from .042, .040, .035 , .032, and .028 to eliminate the possibility of spark blow out. It misses at Idle ( I have an aeroforce scan gauge and it will be anywhere from 2 to 12 misfires at idle) and at WOT now with the CEL flashing.

I have been on this for months guys, MONTHS! and now my old lady is getting ready to get back to school and my car is still not on the road.

ANY IDEAS?!?!?!?!?!
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post #2 of 3 (permalink) Old 08-16-2016, 10:40 AM
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Did you try re-seating the ECM connector or check the ignition coil harness (It may be chafed and touching somewhere)?

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post #3 of 3 (permalink) Old 08-16-2016, 06:36 PM Thread Starter
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Originally Posted by Coby7 View Post
Did you try re-seating the ECM connector or check the ignition coil harness (It may be chafed and touching somewhere)?

Yes ive done that multiple times, about the only thing I haven't tried is pulling out the entire harness and do a deep thorough inspection. Time is being the biggest enemy at the moment for that.

What I am starting to think is this.... I haven't done a leak down test yet but I'm starting to feel its something with the valves. The engine itself only has a little over 43,000 miles (or 69,000 km). Obviously without testing I am only making a rough guess as I have literally tried everything! The place I ordered the engine from probably recovered the engine from a wrecked car, or junkyard, whatever... so how long was the engine sitting before I got it? They say they check the compression before they ship any engine, so I'm hoping that the rings are good.... but do you think the valve seats, or seals, are worn? or rusty (pitted)?

In combination with that, I think the tune is a bit cruddy. Not bashing ZZP on this one per se, but a lot of people tell me they do a very very basic tune. When I pull the plugs, each one is carbon black and fouled all around except the electrode...???? does that lend a clue???

I also want to throw in that I checked fuel pressure and it was a solid 60 psi... was told that a good pump is between 50-60psi... just seems to be a bit on the high side since the pump and the chassis has over 140,000 miles.

And too, just recently, the idle has been like dropping to almost nothing... sometimes below 500 RPM. Then itll go up to 750 rpm, then bounce back down. Thoughts...?
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