rough idle

hi i have a 1991 525i bmw and it has a nasty rough idle. Sometimes the roughidle is almost non existant and other times it is so bad that it makes the car lunge while my foot is on the brake at lights. Some days its good others its horrible. Any ideas?? please help

Check for vaccume hose leak. Check the airmass meter (working) and airmass meter boot for holes.
Finally, check the oxygen sensor. Unplug the oxygen sensor near the air intake (driver's side) and see what happens. It should go to the default level. Your gas milage won't be as good.

Thanks i will check all that out

Thanks i will check all that out

I have been hunting for similar problem in my 89 525i, and tonight just replaced the air mass meter, and ignition wires. Oxygen sensor, idle air control valve, throttle position switch were all replaced recently also. The car will also "surge" and if in gear at a stop light, will rock back and forth if the surge is extreme enough.... After replacing the air mass meter and ignition wires ($450) "ouch", I noticed the idle was still rough. Ahhh!!! Listening carefully I could hear a hiss near the throttle body, but I had thought I had sealed any possible vacuum leaks.... OH NO.... the hiss was coming from the 3-way junction thingie (tech term) between the throttle body, and the power assist brakes. A small nipple on it.... with a perfect little hole... not sure why its there, but putting my finger on the hole stopped the vacuum leak, and the idle smoothed out considerably... I shut the engine off, and put some silicone adhesive on it. I will let you know how it goes tomorrow.... small vac leaks seem to make a big difference in how these things run...

I have heard also that exhaust side leaks can cause problems with idle speed. I am not sure why, but I know that I have a slight leak where the exhaust manifold joins the exhaust system.... Its on the list for fixing, just haven't gotten to it yet.

Also have a similar problem with my M60 engined '93 530. It idles pretty low (about 550rpm), sometimes when I move the steering wheel at idle, the drop in engine speed from the power steering pump causes the engine to drop sub-idle, and the ECU surges the engine to prevent it from stalling. Very annoying. Also have an intermittant rough idle problem too. Also bad oil consumption too. Someone said to me this could be some faulty breather valve located on a manifold somewhere, that could explain these three things?

Any ideas?