Rough Idle

[jim9706]

My 04 GXP has started idling rough and the check engine light comes on (only 12K on the motor). I pulled a P0304 code and that indicates a misfire in cylinder # 4. There are lots of possibilities for the repair so before I go poking around in there I was wondering if anyone has had a similar problem.

[dpnewfie]

Number 4 ignition coil needs to be replaced. Had a similar problem with mine.

[CMNTMXR57]

There isn't much to these ignition systems, so there isn't much to troubleshoot. It's pretty much replace them.

The coil module acts as the coil and plug wire (best term), and it basically bolts onto the spark plug. It uses an internal spring to make sure connection with the plug is consistent and in constant contact. So it's either one of 3 things;

1) PCM feed through the harness telling the coil to fire in it's timing order
2) Coil module
3) Spark plug

But before ruling out ignition as the fault, has the fuel system been ruled out? A fuel pressure injector balance test would probably be the best to determine that all injectors are properly firing. You'll need a Tech2 or other suitable scan/diagnostic tool with bi-lateral controls and a fuel pressure gauge to attach to the fuel rail to run this test.

[jim9706]

Well I finally got the car running right. I took it to a local dealer who still services Pontiac. They checked the plugs and coils and found no problems with either. Then they told me there was a TSB that indicated an engine flush would be needed to remedy the problem. That didn't fix the problem so they then said the car had bad gas" in it so they needed to change the fuel filter. Still no luck. In the end they changed the plugs and that seemed to fix the misfire. Total cost - $518. They did give a $98 credit for the engine flush so the amount I paid was $20 to have the plugs changed!!!!!!!!!!!!!!

I would give up on this car were it not for the very low production numbers and the at thrill i get when I drive it. It's a real love-hate relationship.