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Amedee, Buick Mechanic
Category: Buick
Satisfied Customers: 26265
Experience:  ASE certified Technician advanced level specialist. Wisconsin certified emissions state inspector
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Buick P.A. Ultra Supercharged: I have a*****Ultra

Customer Question

I have a*****Ultra that has a rough idle only after it is warmed up and only for about 10 seconds after you start it.
Submitted: 2 years ago.
Category: Buick
Expert:  Dan replied 2 years ago.
Hello, my name is ***** ***** I'm here to help get your problem solved. Does it run rough every time it starts warm or does it have to sit for awhile before it restarts rough?
Customer: replied 2 years ago.
scenario: car is warm, stop to pick up something so car is shut off for 5 minutes. start car up. After 5 to 7 seconds the engine starts running rough. This lasts for 2 to 8 seconds then car runs fine.
Customer: replied 2 years ago.
when this occurs, as the idle starts to get rough, the vacuum operated waste-gate door opens slowly then closes as the rough idle starts smoothing out.
Expert:  Dan replied 2 years ago.
Have you checked to see if fuel pressure regulator is leaking?
Customer: replied 2 years ago.
No, it is not leaking, fuel pressure is steady around 50psi
Expert:  Amedee replied 2 years ago.
Hello and welcome!
My name isCustomerand I would like to try and point you in the right direction.
My experience has been that this is caused by a bad input to the PCM for fuel control. The ones that would affect the idle the most would be the MAP sensor, MAF sensor, TPS sensor and engine coolant temp sensor. These are the most likely possible causes.
Customer: replied 2 years ago.
any tests I can do to narrow it down? I have no codes.
Expert:  Amedee replied 2 years ago.
You would have to check the voltage from each sensor manually using a voltmeter to see if the sensor is "skewed". You would need to do this WHEN the problem is acting up.
Customer: replied 2 years ago.
Okay, so one of these sensors more likely than the other considering your experience? Which of these sensors fail on a more regular basis?
Expert:  Amedee replied 2 years ago.
The MAF and TPS go bad most often.
Expert:  Amedee replied 2 years ago.
It'sCustomerfrom just answer. I am just checking in to see how things are going and to see if you need any further assistance?
Please let me know!
Customer: replied 2 years ago.
Well, there is a voltage spike at the MAF, then the EGR opens, causing the rough idle. the fuel then tries to catch up the amount of air volume in the plenum(supercharger intake) as the egr then closes the idle smoothes out, rises to about 1300rpms, then settles into an idle at 770rpms. If I disconnect the plug on the egr valve the car runs and operates beautifully except for the cel comes on of course. I have replaced the MAF sensor and the EGR with new units from the GM dealer, but the problem still exists. On a scan I can see a voltage spike at the MAF just before the rough running scenario. I don't know and I cannot detect where the source of the spike is coming from, whether it is from the PCM or in the wiring. But it is not from the MAF sensor itself.
Expert:  Amedee replied 2 years ago.
Ok, then you would have to trace the wiring back to the PCM to ensure that the wiring is good and then possible replace the PCM if all checks out good.