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sprinkles08
sprinkles08, ASE Certified Technician
Category: Pontiac
Satisfied Customers: 21599
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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I have an 03 grand prix autozone said it was a random

Customer Question

I have an 03 grand prix autozone said it was a random misfire. vague! so I changed the plugs and wires and had one new coil pack to swap out between the three , no luck. so I changed the cat converter thinking it was plugged. didn't hurt but didn't fix it! what else could make it run rough even at idle?
JA: Is the rough idle intermittent or consistent?
Customer: consistent
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: yes . plugs wires and CAT
JA: Anything else you think the mechanic should know?
Customer: no
Submitted: 3 months ago.
Category: Pontiac
Expert:  david craig replied 3 months ago.

Hi Im David. the most important thing here is not to start throwing parts at your car. 1. you not sure if its going to fix it.

2. you're wasting your money and time.

3. you dont know what cylinder is misfiring.

Now you said you bought 1 new coil pack and tried swapping them betwwen the 3? there are 6.

Expert:  david craig replied 3 months ago.

it can be a coil pack, a fuel injector, a wiring issue, a bad cylinder etc....

Customer: replied 3 months ago.
the coil packs have two points per pack for a six cylinder which makes three two point packs. and shouldn't the computer throw a code for an injector or wiring issue?
Expert:  david craig replied 3 months ago.

my bad, thinking of a different engine. and no the computer does not throw a code for an injector fault

Expert:  david craig replied 3 months ago.

on high end scan tools you can read injector behavior and actually shut them down and back on.

Expert:  david craig replied 3 months ago.

a generic code reader is very limited.

Expert:  david craig replied 3 months ago.

and depending on if its a wiring issue to the failed cylinder component its only tell you its a misfire

Customer: replied 3 months ago.
ok yeah I wonder how precise they were. what about the throttle position sensor? oh and it was putting of a sulfur smell!
Expert:  david craig replied 3 months ago.

tps not the problem. that would give you a fault. sulfur huh? do a backpressure test on the cat

Customer: replied 3 months ago.
ok I will try that. thank you . and could it be one of the sensors on the exhaust ?
Expert:  david craig replied 3 months ago.

not making it run that bad

Customer: replied 3 months ago.
ok so maybe just getting too much back pressure somewhere then?
Expert:  david craig replied 3 months ago.

yep. clogged cats will do that

Customer: replied 3 months ago.
that's what I thought so I changed it and still no change.
Customer: replied 3 months ago.
what else could be clogged enough to cause this?
Expert:  david craig replied 3 months ago.

hang on a few

Expert:  david craig replied 3 months ago.

sorry man, got really busy yesterday at my shop. Dont take that as your issue not being important. Its hard alot of times to keep up with both. Im looking into this right now.

Expert:  david craig replied 3 months ago.

Do a compression test on all the cylinders. Before we dive into anything else, lets make sure we arent dealing with a dead cylinder. Ill be checking throughout the day for the results.

Expert:  david craig replied 3 months ago.

any progress?

Expert:  david craig replied 3 months ago.

do you still need assistance?

Expert:  david craig replied 3 months ago.

hello??????

Expert:  sprinkles08 replied 3 months ago.

Hello and welcome to JustAnswer!

I'm assuming you had a multi cylinder misfire code set, P0300. This code sets when the engine controller is seeing misfires on more than one cylinder at a time. This could be several things.

Did the problem start not long after getting fuel?

Are yu able to feel a constant miss?
Is there any abnormal engine noise?

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