1998 Cadillac Deville Hesitation and Backfiring

Tiny
LEWIS R
  • MEMBER
  • 1998 CADILLAC DEVILLE
  • V8
  • FWD
  • AUTOMATIC
  • 140,000 MILES
Recently I had a Cadillac garage install a new blower motor for the heater/air conditioner. They had to reposition the engine to make this repair. When moving the engine, is it possible that an electrical connection or a vacuum line might have come partially unhooked. My problem is this now: When I start thecar in the morning, it is a little rough but very smooth after a short time. As I drive away, the car accelerates smoothly and the engine sounds fine and normal. After I reach 40MPH or so and accelerate it, the engine gets very jerky and at times it will backfire. When I am driving 60 or 70 MPH on the road, steadily that is, the car runs very smoothly. If I accelerate the car even slightly the engine hesitates and gets jerky. My first thought was a dirty fuel filter but it would seem that the problem would be all the time. Then I thought about the evap canister. I have only looked for loose electrical fittings and whatever I thought they might have moved when working on the car. I have not looked under the car. Is there anything that could have come loose during that repair or do I have another totally different problem.
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Monday, October 4th, 2010 AT 10:40 PM

3 Replies

Tiny
FACTORYJACK
  • EXPERT
It sounds to me like ignition misfiring under load. There is a possibility that they disturbed the 1, 3, 5, 7 plug wires and they(one or others) are not fully connected. It could also be carbon tracked wires or plugs, causing the spark to find a path to ground. The plug and the plug gap, are rather high resistance, and it is not hard for the spark to chose a path of least resistance. I would say if you don't find something obvious, like #5, or #7 plug wire loose, then replace the plugs and wires. I prefer o.E. Replacements.
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Monday, October 4th, 2010 AT 11:25 PM
Tiny
LEWIS R
  • MEMBER
Thank you very much. Moving the spark plug wires around did work fine for a short time, then started missing and backfiring again. I changed the wires by putting on a new set of wires and everything is fine. I drove over 400 miles yesterday and everything was just fine. I think the backfiring might have been spark jumping from one wire to another wire backfiring on a different cylinder stroke at an input time.
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Thursday, October 7th, 2010 AT 7:32 AM
Tiny
FACTORYJACK
  • EXPERT
Did you replace the plugs as well? If you had a carbon tracked plug, a misfire may return as a result. The track will be noticeable on the exterior portion of the ceramic insulator. It will look like a crack. This misfiring through the boot causes carbon to deposit on both surfaces. As you replaced only the wire(boot), it does not have the track allowing spark to easily take the path of least resistance. If the plug still has that track, sooner or later spark will start leaking and finding a way, and the process will start all over again. If it does, you may also need to replace the tracked wire again. It is just a suggestion. I have had a few that I have just replaced a carbon tracked plug on, that ended up needed a wire to be completely repaired.
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Thursday, October 7th, 2010 AT 11:35 PM

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