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I have a 1989 520i E34 ( 96K miles) . It drives and idles ok (I think) except when the revs are between idle and 1700 rpm. When the revs are in this range the car lurches (jerks) back and forth as if the engine first wants to run at high revs then at low revs then at high revs and so on (hunting engine ??) . It is worst when I am slowing down and so do not have my foot on the accelerator and is very alarming ( it has been getting worse over 18 months).

To describe it another way - when in that rev range it feels like the the accelerator is being repeatedly 'jabbed'.

As above, the problem is most noticeable when slowing down, but I can also 'feel' it when gently accelerating through that rev range (idle to 1700 rpm). I think its also there when crusing at high speed although happening at a higher rate.

When slowing down, I overcome the problem by dis-engaging the clutch, and letting the car freewheel until I need to either brake or accelerate out of that rev range.

I have had nearly all usual service parts changed (e.g. plugs etc) and when I took it to BMW, they diagnosed a pin prick hole in a hose causing a vacuum problem, which they replaced for a small fortune. This did not fix the problem. (PS - on the day I took it to
BMW I could not properly demonstrate the problem !!! ). I have no 'bad' reported codes.

Could someone please help with a solution. People I have spoken to say its a 'vacuum' problem, an 'idle' valve problem. Short of changing half the engine parts would someone please help?

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