Idle issues disappear when AC compressor is turned on

[joncrz]

Hey guys sorry if this is covered somewhere else, the search function kept giving me a "maintenance error". So for a while I have had this rough idle problem on my new (to me) 86 325e (american) with the m20b27 engine. When the engine is sufficiently warm the engine starts to stumble (as if it is just about to die...theeen it recovers) as if it is not getting enough air. My initial thought is that there is a vacume leak somewhere in the system which is not uncommon in such an old car except... the symptoms disappear when I turn on the Air Conditioning on the car. It doesnt matter if the air recycle button is depressed or nor does it matter what other AC buttons I mess with. This is driving me nuts because as far as I have ever figured the vacume system is seperate from the AC system. I admit I havent really dealt with a vacume problem before so I could be wrong.

Any insight as to why my rough idle problem disappears when I turn on my ac compressor ?

[kam-325i]

The throttle is increased slightly by the ECU to take up the load put on the engine by the AC compressor.

You say the idle is rough when the engine is warm, i would look at the blue engine temp sensor.

[joncrz]

I dont understand how a temp switch will reduce the rpms (reduction in voltage or lack of oxygen feeding engine)? are you implying that the ECU thinks the engine is over the acceptable temp range thus cutting fuel (to cool down) causing the RPMS to repetitively dip? If so how does turning on my AC compressor bypass the ECUs effort to cut fuel if all the AC compressor is doing is increasing the load on the electrical system?

I will be testing the blue temp connector mostly because I am running very rich in the mixture and I have trouble starting from time to time... its just hard for me to visualize how the switch will cause the car to idle rough.

[Grrrmachine]

There's a direct wire going from the heater/AC console to the ECU. With AC off the ECU will do nothing if temps rise at idle. With AC on, the ECU increases its fuel input at idle to compensate for the load of the compressor.

The temp switch in the AC system just controls the electric fan. Depending on whether you have a "mono" temp switch or a "hi/lo" temp switch, the increasing resistance of the temp sensor as it warms up will create a corresponding increase in fan speed. Nothing to do with the ECU, nothing to do with the rpms or idle speed.

Your idle problems are most likely nothing to do with your AC system, but the automatic increase in idle speed when the AC system is turned on is masking the existing problem. As kam says, check your Blue Temp Sensor in the engine (nothing to do with the AC system). For more info:

http://www.e30zone.net/e30zonewiki/inde ... emp_Sensor