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What causes this car to run rough from time to time?

(1998 Pontiac Bonneville)
in Saint Louis, MO on November 21, 2011
The car runs like there is a bad spark plug or wire. This is the second engine in the car and it was not new. When I had the computer run on a check engine light, those items were mentioned as possible causes on the number 4 Cylinder as well as an injector. Had the car checked and all plugs, wires, and compression were good on the engine. The mechanic stated that the engine was mechanically sound! The next step the mechanic was going to diagnose was an injector but I did not have the money that day to continue. When I left, the car was back to running as it normally should. Something the mechanic did must have cleared/cleaned the problem out. It has started running rough again and will not clear itself out. Sometimes in the past, the problem would go away on its own. The car will be running fine. You turn it off to park and go do something and when you come out and turn the car back on again, it starts running rough like it has a bad plug. In the past, the problem would work itself out but the last time it did not. It has started running rough once again and will not clear itself out. I know about the plastic intake manifold on this car. I am not leaking any antifreeze as the levels remain constant. Something is fouling an injector as the one cylinder is not running correctly. What does this problem sound like? Do you have an idea what to look for to correct the problem? Also, a referral to a mechanic that is familiar with this problem and who will not charge an arm and a leg or who wants to rebuild the entire engine. A reponse would be greatly appreciated. Thanks!
Displaying 1 answers & 1 comments
Popular Answer
It could be so many things, without being able to experience the problem first hand or do basic diagnostic tests it hard to just guess. Start with the basics, maybe just replace the plugs, read the plugs see if they all look the same. Check for vacuum leaks, after that failing injector or ignition coil could be the cause. Any Check Engine Light on? A professional grade scan tool allows key engine sensor data to be analyzed right off the scan tool without having to do intrusive testing at each sensor.
on November 21, 2011
Thanks for your reply. There is a check engine light on and I had a computer scan done by Auto Zone. There indicated a possible missfire on cylinder #4 as well as a possible injector problem. The spark plugs, wires, and compression were checked on the car. I believe the ignition coil was checked also as the mechanic stated there was not an ignition issue. All the plugs, wires, and compression were good and the mechanic stated the engine was mechanically sound. The mechanic started on the injectors but I felt I was just getting into a lot of diagnostic work at an hourly rate. This problem comes and goes and in the past had cleared itself. When I left the mechanic the last time, the engine returned to running normal and stayed that way for about three weeks and just within the last week started acting up again. Whatever he did in the process of the compression tests helped to clear the problem up. I will go someplace where they have a professional grade scan and see what I can get done. I don't know if this gives you any other ideas but thanks for your reply anyway. If you have any other ideas, they would be appreciated. Thanks again.

Sincerely, Bob
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