I have a 2005 bonneville, 3.8. I stopped for a moment, shut off the car and when restarting, it ideled extreamly ruff and backfired. Without constant accelerator pressure it would die. After a couple of hours of cooling, it started, ran fine for about 15 min and the problem started again. It has never had any engine problem before. I do have an intermittent problem with the oil pressure sending unit dropping out, apparently a problem with this car. I replaced it about 2 years ago, now it is recurring. Any connection?
There could be a relation between the oil pressure gauge dropping off and the rough idling if the problem is electrically related.
Is the Check Engine Light indicating and have you tried to retrieve for trouble codes?
Have the sprk plugs and related components checked.
Ensure there is sufficient coolant in system.
May, 18, 2011 AT 3:39 AM
I don't have a check engine light, coolant seems to be OK and operating temp is in normal range. The car has the original plugs and wires. When it has opportunity to cool, it runs perfectly. Only begins to run rough after reaching normal temp. The oil pressure guage drops to zero and back up to normal frequently. This has been going on for several days, but has been running fine. The oil pressure problem happened 2 years ago and a new sending unit solved the problem then.
May, 18, 2011 AT 12:56 PM
You could be experiencing a failing oil pressure sending again. As to the rough idling, with the milege on your vehicle, the spark plugs are overdue for replacement. Have them checked and replaced if necessary.
When engine is cold, the idling speed is higher so you might not notice if there are any irregularities.
May, 18, 2011 AT 3:25 PM
There is a definite major difference immediatly. It goes from running perfectly at idle and driving to running so badly it won't continue. Hard acceclerator will keep it going (have to get it to 4000-5000 rpm but it backfires. After sitting a while it returns to normal smooth quiet idle. Something is failing at normal op temp. Not a gradual failure, but instant. Could the plugs/wires create this or should I be looking at fuel system?
May, 18, 2011 AT 4:21 PM
I would suggest starting with the spark plugs and ignition components.
May, 18, 2011 AT 7:33 PM
Ok, installed new plugs and wires. Same problem, started and ran like new for about 10 min, then crapped out. Pumping the accelorator is the only thing that keeps in running at all. Still shakes hard and backfires.
May, 18, 2011 AT 8:29 PM
Check the fuel pressure.
May, 18, 2011 AT 8:49 PM
I'm not sure how to do that, I don't think it's fuel starvation, the backfire is coming from the exhaust, not the intake and there is black smoke at rough idle. Before it fails, no backfire, no smoke.
May, 19, 2011 AT 11:54 AM
You would need a fuel pressure gauge to test the fuel pressure.
The additional information indicates it could be a fault with the ignition coil(s).
When replacig the spark plugs, were all spark plugs showing same pattern of combustion or any were different?
Have the ignition coils checked/tested.
May, 19, 2011 AT 5:21 PM
All the plugs were in bad shape, didn't notice any difference. I was able to pull a code. Indicated as you suggested, ignition related. Pulled the ignition control module and replaced, no help. Also had old module tested, came back OK. Could this be one or more of the 3 coils?