Hi all! Hope someone has some ideas on this.
My 92 300e starts and warms up normally at 1100-1200 RPM, but when it gets up to running temp (80C) it idles at 1600-1800 RPM. The Idle Air Control Valve works fine. I can see no cracks or leaks in the hoses to or from it (though they are difficult to inspect). The Coolant Temperature Sensor checks out fine at all temps. The Mass Air Flow checks out fine. The Throttle Valve Microswitch tests fine. The Cold Start Valve checks out fine. I have no DTC Fault Codes. And the car runs fine - except for the high idle.

If I disconnect the plug to the Idle Air Control Valve the engine immediately drops to a normal idle of 650 RPM, but then the Check Engine Light comes on and the Engine Control Unit generates a #18 Fault Code of "Current to Idle Air Control is Illogical", as you would expect. I'm thinking that proves that the hoses to it are OK, but I could be wrong because I know that generation of Fault Codes throws the system into "Fixed Mode" which might compensate for that.

Any ideas?

Thanks in advance.

 

M103 Engine, 124 Chassis. Automatic with AC. VIN: WDBEA30D6NB624256. Oxygen Sensor checks out OK, too.

 

I'm not really an M103 guy, but I would suspect that the airflow pot may be miss adjusted. Here is a link to an article on how to adjust it: http://www.sg-motorsports.com/AFS%20Pot%20write-up.htm. You may need to disconnect the IAC valve temporarily to get it to idle normally while you measure / adjust the mass flow pot.

Also, does you car have the Ecco-junk wiring harness and if so, has it been replaced? Really strange things can happen when those deteriorate.

 

I don't think so. I'll have to ask my mechanic.

A few other things of note:
The car "idles" in neutral at 2100 RPM when coasting down the road until I come to a complete stop. Then it suddenly drops to 1600 RPM. What's wrong here is that the KE system is supposed to cut of the fuel supply to the engine completely if the car is moving and the accelerator pedal is no longer depressed. But in my case, the engine is still revving like crazy. So either the ECU isn't getting the message from the Throttle Switch, it's unable to shut off the Electro Hydraulic Valve or the Fuel Pump, or some other sensor is confusing it so that it doesn't know what it is supposed to do. The bottom line is that the engine is revving when it should be coasting without any fuel whatsoever. So it's not just an "idle" problem. It's a run problem, too.

 

The fuel cut-off under deceleration function only cuts off the fuel flow under engine over-running conditions, e.g., when the manifold vacuum is high and the engine speed is above idle speed. If it cut off fuel while coasting neutral, the engine would die suddenly which would be dangerous. Also, FI systems often keep the idle biased high (but not as high as your car) while the car is moving in order to help maintain smooth operation of the drive train (smoother shifts and less jerkiness on throttle movement). The behavior you describe seems normal except the the actual engine speeds are too high.

As a side note, I built an FI system from scratch for my Triumph TR8 and spent a great deal of time programming the parameters around these two functions in order to get the fuel cut off to happen imperceptibly. I set up the idle control to maintain 900 RPM in neutral while the car is moving and then drop to 675 RPM when stopped.

 

Check vacuum lines also !

 

Thanks for all your help guys. I've never even changed a plug or the oil on this car, and I know absolutely nothing about electronic fuel injection (much less which variety this car actually had), so I had to start from scratch.

My mechanic did everything he could think of, and he came up with zero. That was a $500+ owie. So I figured I'd have to learn how the whole system operated, which I did after much research, studying, and thinking. Everything pointed to the Throttle Valve Switch - NOT the Throttle Linkage Microswitch which is commonly confused with it, the VALVE Switch which sits below the Throttle Butterfly Valve. So I bought an ohmmeter and tested it (along with all the other stuff you have to test to make sure it's all OK), and sure enough, that was it. It's supposed to be <1 ohm at idle; it varied between 77 ohms, 18 mega ohms, and infinity - all while just sitting there. That should have been the clue, but I'm new at this game and a slow learner to boot.

So I called my mechanic to see if he had a switch since I couldn't find one on the internet. He did (undoubtedly used), but he couldn't install it for a few days because he's so busy. So just to make it easier for him when I bring the car in, I used a toothbrush and some Simple Green to clean all the gunk away from the area. I got it nice and clean - and then it worked perfectly.

So the problem, obviously, is A LOOSE WIRE to the Throttle Valve Switch. Since it's working now, I'm just going to use it. Meanwhile, I'll see if I can dig up another switch just for insurance. If I can see the darn thing out of the car I'll have a better idea of what is involved in getting it out to replace it.

Thanks again for all your help and encouragement. Without all the resources these blogs have led me to I never could have found the wiring diagrams, part and code numbers, explanations, and all the rest I needed to figure this thing out. It really was a valuable, valuable experience.

Right now my beautiful, IMMACULATE car runs like it is brand new. I'm hoping I can keep it that way for a few more years. I just LOVE this car.

 

The Throttle Valve Switch must be the sensor that tells the ECU when the car is truly at idle or not, and therefore, whether to be in closed or open loop idle speed control operation. In the above mentioned TR8 FI system, there is a throttle position sensing potentiometer that reports a voltage proportional to throttle position. You specify firmware what voltage corresponds to an idle throttle position. In the MB it appears to be a simpler on/off binary function.

Glad you found the problem!

 

Yes, it is a simple on/off switch: less than 1 ohm at idle, infinity at run. Thanks again for your interest and help!

 

I had a very similar, nearly identical problem with my 1990 300E.

I took the air cleaner off to look for the Throttle Valve Switch as mentioned earlier. What I found was that my throttle cables plastic casing was sun baked, gone in spots, and disintegrated to the touch. (I live in Arizona, plastics don't fare well here in general.)

Where the metal cable was exposed the metal looked corroded. A heavy dousing of 3 in 1 oil on the exposed cable, the return spring as well as anywhere that moves in the throttle assembly. (like the detent cable)

Thus far that seems to have cured the problem. I plan on replacing the throttle cable in the near future.