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Fails to idle

Tiny
MIKETURNER55
  • MEMBER
  • 2005 PONTIAC BONNEVILLE
  • 6 CYL
  • AUTOMATIC
  • 123,000 MILES
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?
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Tuesday, May 17th, 2011 AT 6:26 PM

14 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
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.
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Tuesday, May 17th, 2011 AT 8:49 PM
Tiny
MIKETURNER55
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  • 8 POSTS
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.
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Wednesday, May 18th, 2011 AT 3:39 AM
Tiny
KHLOW2008
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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.
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Wednesday, May 18th, 2011 AT 12:56 PM
Tiny
MIKETURNER55
  • MEMBER
  • 8 POSTS
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?
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Wednesday, May 18th, 2011 AT 3:25 PM
Tiny
KHLOW2008
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I would suggest starting with the spark plugs and ignition components.
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Wednesday, May 18th, 2011 AT 4:21 PM
Tiny
MIKETURNER55
  • MEMBER
  • 8 POSTS
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.
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Wednesday, May 18th, 2011 AT 7:33 PM
Tiny
KHLOW2008
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Check the fuel pressure.
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Wednesday, May 18th, 2011 AT 8:29 PM
Tiny
MIKETURNER55
  • MEMBER
  • 8 POSTS
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.
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Wednesday, May 18th, 2011 AT 8:49 PM
Tiny
KHLOW2008
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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.
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Thursday, May 19th, 2011 AT 11:54 AM
Tiny
MIKETURNER55
  • MEMBER
  • 8 POSTS
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?
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Thursday, May 19th, 2011 AT 5:21 PM
Tiny
MIKETURNER55
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  • 8 POSTS
Tried a new coil module in all three positions. No help, unless mutiple of the units failed at once. Dosen't seem likely. What next?
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Thursday, May 19th, 2011 AT 7:15 PM
Tiny
KHLOW2008
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What code did you retrieve?
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Thursday, May 19th, 2011 AT 8:49 PM
Tiny
MIKETURNER55
  • MEMBER
  • 8 POSTS
It was showing ignition control. I gave up and limped it over to a shop. It was the MAF air flow sensor. It was reading massive air flow even with the engine off, then it would fluctuate when the engine started causing erratic fuel mixture, mostly way too rich. Replaced that and drove it 25 miles without incident. Was a tough one. Thanks for all your help.
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Thursday, May 19th, 2011 AT 11:54 PM
Tiny
KHLOW2008
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You're welcome.

Have a nice day.
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Friday, May 20th, 2011 AT 11:14 AM

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