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Hello all,
I've been trying to track down an idle issue on an '89 740 Turbo (LH 2.2). Years ago my IAC failed and I had a rolling idle. However, since the mechanics touched the car and replaced the IAC it's never really worked right since.
The engine does not compensate when the A/C is turned on (just drops the idle a bit). Before the failure years ago, the engine would clearly compensate (would idle slightly higher when A/C was on). Unplugging the IAC connector, the engine does not run any differently. Clamping one of the IAC hoses also does not change anything.
I've done the other following tests so far:
- TPS is working 100% correctly. Took the throttle body off the car, cleaned well, tested the TPS pins and are to spec. Ensured the TPS connector is good (replaced connector), engine definitely gets the TPS idle signal (can tell a difference in how it runs connected vs not connected).
- Grounding the CIS test point does not change anything with how the car idles.
- IAC valve is functioning electrically. Took off car, checked it is clean inside. Powered the middle pin, grounding either of the outer pins causes the valve to open/close respectively.
As a final test, I made sure the IAC was in the open position and I put it back on the car but without connecting the IAC connector. I got a rolling idle (800 - 2000 rpm) until I reconnected the IAC connector, at which the engine went back to how it's been idling before.
It seems that the IAC is only being given a "close" signal from the ECU but not an "open". I read a post from years ago about transistors going bad in the IAC control loops within the ECU (in their case they only got an "open" but not a "close" signal). In that owner's case, they were able to replace the transistors to fix the problem.
Are there any other obvious tests to check first before I repair or replace the ECU? Would appreciate any help on this.
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