Erratic idle

[mirokus]

Hi There,
I have 10/88 build E34 5 series which has fluctuating idle.
when first starts (cold) it runs fine for about one minute than it starts to fluctuate between 800-1500 rpm.I have eliminated Air flow meter, ICV valve,vacuum leaks,engine breather system, ignition system,fuel pressure and regulator. System has been scanned with Hannatec scan pro and no fault was found. When engine is running if ICV wiring is disconnected from the valve, slight knock on ICV with screw driver will bring idle down and car will run perfect until ICV wiring is connected then it starts to fluctuate again. Any advice regarding this problem would be greatly appreciated.

Kind Regards,
mirokus

[Kalevera]

What leads you to believe that the ICV isn't the problem?

best, whit

[mirokus]

What leads you to believe that the ICV isn't the problem?

best, whit

I have tried 2 other working ICV valves as well as Airflow meter.

mirokus

[genphreak]

Check TPS - your idle control valves may well be working, but without the signal to turn them on then they wont do anything....check your throttle position sensor, if that doesnt activate the microswitch when you let off the throttle the ICV wont switch on - causing your erratic idle - they tend to either break or dont get touched becuase the return arm is out of adjustmentbtw; with the engine off, you can hear the switch engage when you move the throttle (with your hand turning the throttle pulley on the throttle body) back to its rest position. It should click inside the switch housing (mounted on the throttle body in case you didn't know where it is) audibly, literlally the moment it returns to rest, -ie dead-on, not before. To adjust it is as such read the Bently, adjusting the mounting screws ever so slightly so the switch activates dead on the idle position. :) Nick

[mirokus]

Check TPS - your idle control valves may well be working, but without the signal to turn them on then they wont do anything....check your throttle position sensor, if that doesnt activate the microswitch when you let off the throttle the ICV wont switch on - causing your erratic idle - they tend to either break or dont get touched becuase the return arm is out of adjustment

Thank you for your imput. Regarding TPS I would expect Diagnostic scantool to pick up the fault I have scanned the system with 2 different scantools. The vehicle has been driven for about 25km. without logging any fault codes in ECM. Does this vehicle have seperate ICU (Idle Control Unit) or is it part of the main ECU? What is the correct procedure to set base Idle? 3 wire on the ICV, how can I test voltages or signals and what shoul they be?
mirokus

[mirokus]

btw; with the engine off, you can hear the switch engage when you move the throttle (with your hand turning the throttle pulley on the throttle body) back to its rest position. It should click inside the switch housing (mounted on the throttle body in case you didn't know where it is) audibly, literlally the moment it returns to rest, -ie dead-on, not before. To adjust it is as such read the Bently, adjusting the mounting screws ever so slightly so the switch activates dead on the idle position. :) Nick

Thanks Nick. One thing I didn't mentioned not sure if its wright or not as I dont have workshop manual. As soon as Ignition is turned on ICV comes on and stays on ,you can hear it buzzing. I disconnected TPS wiring from the switch and ICV was still on. If TPS is faulty would you expect 2 different Scantools to pick the fault up?
mirokus

[genphreak]

Thank you for your imput. Regarding TPS I would expect Diagnostic scantool to pick up the fault I have scanned the system with 2 different scantools. The vehicle has been driven for about 25km. without logging any fault codes in ECM. Does this vehicle have seperate ICU (Idle Control Unit) or is it part of the main ECU? What is the correct procedure to set base Idle? 3 wire on the ICV, how can I test voltages or signals and what shoul they be? mirokus It is all in the Bently manual ($52 odd), I haven't the time to transcribe it atm. Sorry! I don't know how the diagtool would know if the TPS is working or not- how would it know the car is at idle and not just always driving on throttle? But for all I know it might be smart enough to check for a closed TPS response at some point...

Setting the idle is a matter of checking everything (else) is within tolerance and then adjusting the AFM flap rest point at the intake side. You really need to measure the AFM ouput signal (at least that it is correct at rest) before this and be ready with an O2 sensor to set the idle air bleed (AFM air bypass) grub-screw too- but htis is optional (makes a very small difference). I hope you are sure you are not dealing with an air-leak, they are sooo common, mine was in the oil-sump gasket!

[mirokus]

It is all in the Bently manual ($52 odd), I haven't the time to transcribe it atm. Sorry! I don't know how the diagtool would know if the TPS is working or not- how would it know the car is at idle and not just always driving on throttle? But for all I know it might be smart enough to check for a closed TPS response at some point...

Setting the idle is a matter of checking everything (else) is within tolerance and then adjusting the AFM flap rest point at the intake side. You really need to measure the AFM ouput signal (at least that it is correct at rest) before this and be ready with an O2 sensor to set the idle air bleed (AFM air bypass) grub-screw too- but htis is optional (makes a very small difference). I hope you are sure you are not dealing with an air-leak, they are sooo common, mine was in the oil-sump gasket!

Thank you for your help.
mirokus

[mirokus]

Thank you for your help.
mirokus

Thanks for your help guys. Problem fixed faulty ECM.
mirokus