Here's a mystery

[allbmw]

I am a new member of this forum and have enjoyed reading the posts. It seems like there are a lot of experienced members out there.

Here is a problem that has been driving me crazy for quite a while, but I have been unable to fix in spite of a lot of effort.

Car runs great when cold.
Warm, with the throttle slightly depressed, the tach needle bounces up and down 100-200rpm in the 2,000-3,000rpm range. In sync with the tach is a miss. It is a soft miss, although noticeable and I feel it is fuel rather than spark generated, but I am not an expert. It appears as an intermittent bump while driving on the highway.

Step on the gas and the bouncing stops and the car accelerates well, although I think that there is a slight surging. Ease off the gas and the bouncing stops.

I have been through all the usual suspects: grounds cleaned and tightened, new rotor, cap plugs and impulse generator,adjusted the gaps on trigger wheel, no vacuum leaks, new ignition control unit, timing set, fuel injection module is good(I have a back-up I tried), temperature sender tests good, new oxygen sensor.

I would appreciate any help you might give me in this matter.

Alan

[Selfish]

did you check/replace your fuel filter?

[allbmw]

Yes, quite recently. I think that the problem may be in my air flow meter. I am going to check it out this weekend.

Thanks for the suggestion,

Alan

[scarceller]

Alan, did you ever figure this issue out? I have same problem in my Motronic 84 911. At around 2500-3000RPM the tach bounces and I can feel a miss in the engine.

I know the tach gets it's signal from the Reference sensor on the flywheel so I'm thinking that a Ref Sensor malfunction, like failing to trigger once in a while would cause this. It also most likely would cut spark as well.

In my car this tends to act up in hot weather if the car just sits in the heat all day not running. When I start the car all seems well, it idles fine. But once I take off down the street the issue starts to act up. After 10 or so minutes it goes away.

Did you ever get to the bottom of this in your car?

Thanks.