Hi:

Have a 1997 Bonneville SSE that continues to throw a P0305 code. Has new plugs/wires.

While troubleshooting, swapped plugs, wires, coil packs, fuel injectors and trouble code stays at number 5 cylinder.

Plug looks good. Compression test to 190.

Cleaned grounds under drivers foot/door panel.

Only occurs upon extended idle, on a warm engine. Does not occur when driving.

Any suggestions? Possibly an ICM? Never had one of those go out before. Everything else on vehicle works excellent.

Thanks.

Dan in MN
1997 Bonneville SSE

I don't suspect the ICM, not on a single cylinder misfire. You already changed plugs, wires, coil and injector. That covers all the easy problems. After that point you need to start looking at mechanical issues. Lucky for you it'* the front head, so removing the valve cover is easy(er) Inspect the valve springs. At this point I would suspect a broken exhaust valve spring. Although I don't suspect this problem, it might need to be addressed. Have you ever had a damaged spark plug in that cylinder? A chipped piston can also cause the problems your having. You will need a boarscope to check this.

Broken valve spring.



Chipped piston.

Thanks for the response. Maybe mechanical is next to check. Engine runs very good, no noise, etc...Also, only throws the code at extended idle. Can drive the car for months without throwing a code, but if left idling for 10 minutes when at operating temperature, then has a random possibility of throwing the code. If not electrical, then will need to check into the mechanical. Compression was good, but who knows...

Thanks.

A broken spring will drive normal, but will sputter under heavy load. A chipped piston will drive normal but you will see a lot of knock retard. You will need a scanner that covers enhanced codes for that.

Thanks. Will check it out and post back if I can figure it out. Not in a rush as it is presently not very often. No doubt will become more often which might indicate what it is...

We have seen someone that had a bad ICM causing a single cylinder misfire. It was one out of billions of cars, possible though.

I would think a broken spring would allow the valve to stay down a little and that compression wouldn't be at the nice 190 that you saw.

When I did a compression test on both broken springs I have seen the test came back perfect. It wasn't until we removed the valve cover that we found the broken spring.

I'll pull the valve cover and check it out this weekend. Supposed to be a rainy weekend in MN so a good time to do so. Thanks for all the suggestions.

Okay. Pulled the valve cover this weekend, and all looks good. No broken valve spring that is visible to the eye. Also redid the compression test, and realized I must have tested incorrectly previously. Cylinders 1 and 3 are about 178 and cylinder 5 is approx. 168. Also exchanged the ICM with a known good unit. Restarted the car, and runs good/normal. Test drives okay. Decided to let idle, and after 15 minutes it triggered the P0305 code again. After clearing, would let it idle again, and after 5-10 minutes at idle would again trigger the P0305 code. After clearing again, drove the car approx 90 miles and up to approx. 100 mph. No codes triggered. Let car idle in driveway, and after 5 minutes triggered the P0305 code again.

Maybe I just can't let this car idle anymore? Okay, so replaced or exchanged nearly everything except the computer and short of pulling the head off to inspect piston/valve issues, which reluctant to do for what appears to be a minor issue caused at extended idle.

Appreciate all the previous suggestions and to date have been able to remedy every other situation on this car in-part with knowledge gained from this forum. Any other suggestions appreciated however may be something that will need to be tolerated on this vehicle.

No need to pull the head. See if you can rent a borescope. A chipped piston is a possibility, but I would expect problems under load not idle. Worth a look though. The lower compression on that cylinder could be the problem. A burnt or sticking valve.