I have a 1987 bmw 325i and it idles rough high and low and before i take in for diagnostic test was wondering what it could be so i dont get screwed at mechanics!
I had a similar problem on a 318IS.
The rough idling was accompanied by an occasional whistling noise from the general engine area (caused by air leaks through the front and rear crankshaft oil seals).
The problem was caused by a faulty crankcase ventilation valve (Costs about $100).
The vent valve is easy to replace - (2 screws) - check the vacuum hose that connects this valve back to the tappet cover air vent nipple - it goes soggy and disintegrates.- replace with suitable oil resistant vacuum hose.
While on the job, clean the throttle body / butterfly valve internally with a soft toothbrush dipped in petrol. This cleans off sooty deposits - makes engine idle rpm more consistant.
On the 325 series I have worked on (your year, make, model), I have found that idling issues like you describe are the result of a defective oxygen sensor. Replace the oxygen sensor with a Bosch unit. That should take care of your issue.
Replace the air flow meter. I had a simular problem. Relaced the air flow meter, the car runs fine.
Check for vacuum leaks. around intake .pvc valve .
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