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Need help with a P0300 code

derpyimp

derpyimp

New member
Hey guys, I could use a few ideas on this one. My wife drives a 2005 Chevy Impala with the N/A 3800, and it started misfiring yesterday. Today it got worse and started throwing P0300 for multiple/random misfires. I test-drove it and I can only get it to misfire at about 75-100% throttle, and even then only some of the time. Took maybe a dozen hard acceleration runs tonight before I got it to show up. My wife said it misfired more when she was accelerating too. The car still pulls as hard as ever, but you can feel it stumble when the misfires begin. I have scanned it and found that it is misfiring on all six cylinders when it happens. There is no problem with rough idle or hard starting.

My first thought was fueling since it showed up on all cylinders and only under load, so I tested the fuel pressure with key on/engine off, at idle, and while revving it. Everything looked to be within spec and never dropped below 46 psi. Pulled the vacuum line off the fuel pressure regulator and found no gas leaking.

My next thought was ignition. I replaced the original spark plugs with NGK TR55 plugs about a year ago, so I pulled two of the front plugs to check for fouling, thinking maybe it was too cold a plug. They looked normal though. I ran the car at night and used a spray bottle to get water mist around the plug wires to help look for arcing, but found nothing.

The only other ignition problem I've had was also about a year ago when I stupidly sprayed water on my hot engine at the car wash, which killed one of my coil packs. I replaced it and also replaced the other two recently for good measure, so all the coil packs are new. The only major work I've done on the car recently is replacing the transmission, which I did myself a few weeks ago.

I'm unsure where to go from here. I know there are many things it could be, but I can't afford to throw parts at it right now (spent all my money on the tranny). Any suggestions?
 


Based on your info my best guesses would be either dirty TB/MAF, clogging cat, or possibly a bad ICM.
 
if you have a scanner, I would scan the LTFT's to see if they go to 0 under wot and what they are during normal driving.
 
sounds like plugs and wires time to me. go back to a iridium plug like it should have, after all your just n/a.

or you got a clogged cat.
 


being n/a, im all over that cat being clogged up. how many miles she got? its kinda the na engine deal. they tend to kill cats more so then the sc set ups.

the check engine light flashing when you floor it? if so id lean towards the cat, or needing a tune up real bad. if you got a temp laser you can check the cats temp, pre and post cat after taking it out for a spirited run, or at night beat on it some, pull over look to see if the cat is glowing red.
 
Just over 130k miles. My wife said the check engine light flashed at her yesterday, but then the car starting acting normal again for the rest of the day. I also haven't seen it happen during my test drives. And it doesn't feel like a clogged cat because when it doesn't miss, like 9 outta 10 runs last night, It pulls hard all the way to redline. The misfire seemingly comes and goes.

Anyway, I just cleaned the MAF, so I'm gonna go drive it again and see if it helped anything.
 


sounds a lot like a busted up cat, inside breaks up, clogs and then can clear its self up when you hit a bump in the road.

get under the car, ( when cold) hit the cat with your hand, listen for it to rattle inside.
 
I have a used catalytic converter that came off my 2001 GTP, might be able to meet up tomorrow sometime if you are interested. I would sell it for 50 bucks.
 
I'm not seeing a bad cat. A flashing cel per gm manual stays " possible catalyst damaging condition", meaning the pcm sees the misfire and is warning you. A plugged cat wouldn't act up only when up to temp, it would act up all the time.

You say it's misfiring on all cylinders, you actually looked at the misfire reporting on each cylinder and confirmed that right? Do you have the ability to do a crank learn? I'm leaning toward possible crank sensor or icm, maybe coils.... but coils shouldn't appear on all cylinders. You are looking for something that would do something to all.

Any work done on the car in the last say year?
 


SgtMarshal said:
I have a used catalytic converter that came off my 2001 GTP, might be able to meet up tomorrow sometime if you are interested. I would sell it for 50 bucks.
Click to expand...

I went and hit the catalytic converter a few times and didn't hear anything unusual that might have been shaking around inside. If it ends up being the cat though, I would probably take you up on that.
 
BillBoost37 said:
I'm not seeing a bad cat. A flashing cel per gm manual stays " possible catalyst damaging condition", meaning the pcm sees the misfire and is warning you. A plugged cat wouldn't act up only when up to temp, it would act up all the time.

You say it's misfiring on all cylinders, you actually looked at the misfire reporting on each cylinder and confirmed that right? Do you have the ability to do a crank learn? I'm leaning toward possible crank sensor or icm, maybe coils.... but coils shouldn't appear on all cylinders. You are looking for something that would do something to all.

Any work done on the car in the last say year?
Click to expand...

I had Torque running with misfire histories graphing for each cylinder individually, and all of the graphs spiked when it misfired. So yes, I know it is all of them.

I do not currently have the ability to do a crank learn myself. Local dealership charges $99 for it.

Everything done in the last year:
-New spark plugs
-New plug wires
-New ignition coil packs
-New remanufactured transmission
-New front sway bar
-New oil pan gasket
-New brake pads all around
-U-bend delete with new rear O2 sensor (about two weeks ago)
 
did you happen to keep your old coils? Maybe try to swap one on and see if those misfires die down on those cylinders. MSD coils for the longest time were causing misfires at higher rpm within a year of being new as well. Something just seems off.
 
I think I got rid of them, but I'll look around and see if I kept them somewhere. If not, I could swap over coils from my Comp G and see what it does.

Btw today test driving, it misfires a lot even at light throttle. It's weird because it's not very consistent. Sometimes it's really bad, and sometimes it hardly misses at all.
 
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