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Recently my '91 Bonneville has been having an issue with idling while stopped, which may or may not have coincided with Service Engine Soon light turning on. The main symptom is an overall drop in the idle RPM (100~300 based on the tachometer), but this symptom will frequently be aggravated when attempting to accelerate (such as from a stop light/sign), often to the point of creating a noticeable stutter and every so often a stall.

In the past I would only notice this type of behavior when the tank would be down to the last gallon or two of gas, and then typically only when turning while accelerating from a stop (such as turning out of a parking lot).

Regarding the Service Engine Soon light: I took the car to a local shop to have the ECM read but their scanner was unable to connect to the interface. They did however replace some rotten vacuum hoses, which did not appear to have a noticeable impact on the idling issue.

Any suggestions on what I should check would be appreciated.

Update:
I was able to retrieve the error code stored in the ECM. The error number was 41, which according to my repair manual may be one or two of three things (my car is fuel-injected):

  • 41- no distributor signals to ecm, or faulty ignition module (carbed models)
  • 41- cylinders select error- mem-cal or ecm problem (fuel-injected models)
  • 41- cam sensor circuit (3.8 engine)
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2 Answers 2

I had a similar problem on 2 vehicles, a 03 mustang and a 99 bonneville. When the car would come to a stop light the engine would idle too low and stall out. The problem (in both cases) was the mass air flow sensor. Its a piece of electronics that determines the amount of air to let in to the engine from the throttle based on inputs. You could test it by throttling the car while on idle to see if you still get a stall.

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I tried as you suggested and when I rev the engine while stopped it does not seem to stutter or stall. I tried both while stopped and in Park (cold and warm) and at multiple points on a 15 minute drive (stopped in Drive with one foot on the break). I was able to retrieve the code stored in the ECM, the details of which I have added to the question. –  Xantec Apr 28 '11 at 16:06

You could try changing your fuel filter(s) and PCV valve if your car has one.

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