Multiple missfire help

**L36pro** · 04-28-2011

my aunt has an 03 gp gt. its throwing a p0300 multiple missfire code. it has every ounce of 30 hp at WOT. theres hardly any acceleration. it was eating coolant so i put a new plenum on it and took it for a drive. still misses. i swapped the ICM out of my 98 into hers and there was a slight improvment in power. checked the resistance on her coil packs which checked fine. i put new plugs and wires. slight improvement. its not throwing the check engine light. at WOT it takes forever to hit 35 mph. it will top out at 4,000 rpm and stay steady for about 7 seconds until the rev limiter kicks in and you have to back off the throttle. i dont have an Fing clue whats going on. im gonna put a new fuel filter on it tomorrow.
any ideas? this is my aunts daily driver and i dont want her to have to take it to a shop thats gonna ream her on the fix.
thanks for any help!

**L36pro** · 04-28-2011

flashed the pcm. didnt help

**Jaybird** · 04-29-2011

Check transmision fluid.

**gtpinsc** · 04-29-2011

transmission fluid for misfires?.... LOL ... how did the old plugs look?

**BillBoost37** · 04-29-2011

I'd want to see how it runs etc. But MAF, clogged cat, bad O2 are my first impressions.

**L36pro** · 04-29-2011

plugs werent too bad. clearly old. trans fluid level was fine

**L36pro** · 04-29-2011

clogged cat is my impression. she said it didnt start acting weird until right after she took it to a shady muffler shop. i checked all the exhaust lines for signs of tampering but didnt see much. it makes sense that they would purposely clog the cat to get her to come back to them. im not sure what else you could do in an hour to make a car run like crap. but they tried to railroad her for a new serpentine belt for $90. when she told them no they got pissed.

**BillBoost37** · 04-29-2011

Right...blame the exhaust shop. If you have more info...please realize that holding it back isn't going to help us diagnose the issue.

did the exhaust shop do? Because you saying taht you looked at "exhaust lines" cracks me up. There's one pipe.. it's tough to screw with one 2.25" pipe to cause trouble.

**L36pro** · 04-29-2011

by lines, i mean routing. and by look i mean i checked all the outlets, all the gaskets, unbolted the downpipe and check down there. and i didnt blame the exhaust shop. im just telling you that in the parking lot of the exhaust shop that tried to screw her, her check engine light came on. i had it taken to Another shop and the cat is totally clogged. and someone could easily unbolt the downpipe and stuff something in there. ill know tomorrow when i take it apart.

**matt5112** · 04-29-2011

$90 for a belt is normal here.

labour + the belt would be around that.

I removed all of your slights against bill.

But next time I wont be so lenient.

This is your second time and I believe your second warning.

**TheOtherNick** · 04-29-2011

Well if you where half as smart as bill you would have tried driving it while you had the dp loose to see if it ran better to see if the cat was clogged.

**L36pro** · 04-29-2011

right. so you guys can be as rude as possible, but i cant say anything because youll kick me off the forum. hooray for internet intimidation.
and nick, unbolt the downpipe and drive through my town. see how long it takes to get a fat ticket.

**Jaybird** · 04-30-2011

Originally Posted by L36pro

at WOT it takes forever to hit 35 mph. it will top out at 4,000 rpm and stay steady for about 7 seconds until the rev limiter kicks in and you have to back off the throttle.

power not getting to the wheels? transmission. this was the basis for my earlier comment.