Check Engine Light

Hi, was wondering if anyone could tell me what would be causing a check engine light im getting in my 1991 735i. It is idling rough (about 600rpms) and the check engine light comes on. When i bring the rpms up to about 1500 the light goes out. The light will also come on during normal driving if the rpms drop below 1500 or so.

Any info would be appreciated.

How is the voltage at idle. Try this with engine idling press 1000 and 10 on OBC simultaneously, this should give you a message on the OBC that says TEST . Then scroll down using the 1 key on OBC until you reach 9 and then press the set reset (SR ) button and this will show the alternator voltage, voltage should be between 13.4 VDC and 14.1 VDC, if not the alternator is not putting out sufficient voltage at idle. You can leave the OBC in this mode and rev up engine, or drive off and monitor your alternator voltage. You can check battery voltage the same way but with engine off and key in position #2 (run).

Thanks, I wasn't even thinking that it might be a voltage issue. I thought maybe some type of emissions problem. O2 sensor etc.

These cars are very sensitive to voltage fluctuations and a few volts low will set off a CEL or other prompts. My 1988 750 and was doing similar issues to your car and it was the alternator that was going (low voltage at idle) and finally the alternator died. Replaced alternator and no more issues.

I have also been getting prompts telling me i have a brake light malfunction when none exists and or a coolant level problem. Hopefully it is something as simple as an alternator.
Thanks for the info.

Coolant issue could be as simple as a bad coolant level sensor (that part that screws into the top of the expansion tank). Brake prompt, is it brake lining? if so then the pad/s may be worns and the brake pad sensor (located in left from outboard and right rear inboard pad) m,ay be telling yo it is time to chnge the pad/s.

One more thing, check the alternator belt tension, if belt is loose this will cause a voltage problem.

Finally go to:

http://www.bmwe32.masscom.net/ for more DIY information.

Or just http://bmwe32.masscom.net/

Sounds like what I just went through, have you tried the stomp test to get the engine codes and then gone to the web site to see what the code or codes mean, I tried, but the stomp test didn't work for me, either that or I don't know how to floor an accelerator. So my issue was drop it from drive to 4th, try to pass someone cel came on, went out, did it again, cel came on and stayed on. Ended up being too much air (Lean mixture) and a missing hoseclamp fixed it for a day.. The real culprit was a bad fuel pump, so 2 fuel filters and a fuel pump later and she is running sweet. There is a link rough idle @ http://www.e38.org/e32/rough_idle_and_shimmy_ultimate_guide.htm
I asked my mechanic to check a few items on the list including the fuel pump and that's what it was. So yes check the alternator to make sure you are getting spark and also check the pressure to see if your pump is going dead.