Idle Control: M104

[latief]

I was thinking about the slightly fluctuating idle on my car, and i am wondering which part is responsible for idle control? their is no idle control sensor or valve on this car, is that correct? what is the part responsible for this?

thanks,

[Kestas]

It is the electronic throttle actuator (ETA). This is a drive-by-wire design. The 93 M104 is at the cusp of having the dreaded underhood wiring insulation problem, which also plagues the ETA. Your first step is to unwrap as much of the wiring bundle to the ETA as you dare and check if the wire insulation is brittle.

[babymog]

There is only an ETA if you have ASR (traction control) IIRC.

Does the idle fluctuate with some regular rythm (hunting)? If so, it is likely running lean due to a vacuum/air leak (common).

[latief]

The idle is fine in general, engine is nice and smooth. but occasionally on the traffic light while in D, there is a very subtle fluctuation (hardly noticeable). i only notice it because i look at the rpm all the time (i'm obsessed)...

another problem that might be related is that the car sometimes seems to surge out of no where while slowing down to a stop. it seems like it drops a gear and surges forward. it is dangerous because if i don't have my foot on the break, it might ram into the car in-front of me...

could a tranny related vacuum issue cause both? or is the infamous throttle wiring harness degradation issue?

Thanks,

[babymog]

If you do not have traction control (ASR), you have a cable-operate throttle.

Could be the engine needs a tune, starts to miss a little and the idle control over-compensates.

[Kestas]

My model (95 E320, M104) does not have traction control, yet it has an ETA.

latief, like I said, it could be the wiring insulation. You'll have to check to rule it out.

[babymog]

I have two '95 E320s also, an early and a late. The ETA that you're seeing is the cruise-control actuator, I believe that it does nothing when you drive on the pedal.

[latief]

I have metal wire mechanism for the gas pedal, but i also have the throttle body with a huge (potentially corroded) wiring harness.

As for the tune up, the engine has had a head gasket done recently as you might remember, all new spark plugs, wires, some sensors, hoses,,,,etc, not much more to change!

I am doubting something in the transmission, because it shows signs of oil..not sure what the name is, but it the vacuum line connection at the tranny side....

[Kestas]

Babymog, there is no direct mechanical connection from the cable coming from the gas pedal and the throttle butterfly on my 95 E320. This makes it an ETA (drive-by-wire). And yes, it is also part of the cruise control unit.

[latief]

A0001415725 = Cruise Control Actuator (used without ASR) ...

That is from EPC, up to year end 1996

But, theoretically, a corroded wiring on this will cause problems since it can open/close the throttle, correct?

[Kestas]

What I'm talking about is not corroded wiring but crumbling insulation. When the insulation disintegrates, baer wires then touch each other causing spurious shorts and driveability problems. In bad cases this bad wiring can ruin delicate and expensive electronic components. So if you do find disintegrating insulation, it's be wise to fix this problem before doing anything else.

[PhantomLord]

Throttle controller cable are badly effected by insulation disintegration problem, only noticeable if you split the outer cover of the cable. But they mostly wont fail unless cable is badly twitched as cable outer is pretty thick and wire inside are held very tight. In case of malfunction ECU should register a error code.

Throttle controller should be the last thing to check for rough idle issue.

Attached is throttle controller cable split from M111 engine, you can repair it but its not long term solution, I think there is DIY article on repairing it here.