1991 300E, typical intermittent high idle (~1,800 RPM). I performed quite a few searches over the last few weeks, and already ruled out my microswitch and throttle linkage. Based on observations from others, last night I finally disconnected the Idle Control Valve while the car was idling high. Immediately the idle dropped down to 650 RPM and was smooth as silk. I drove the car around town for a few minutes, and boy was it NICE! I decided to leave it unplugged and enjoy the idle the way it was meant to be.

After leaving the car parked for a couple hours, I went outside and started it. It could barely keep running. Though relatively smooth, idle was about 450 RPM, headlights and dash lights were very dim. Turning off the A/C didn't increase the idle, it just smoothed it out a little. I think if I put it in gear it would have died for sure. And the engine wasn't even what I would call "cold" - there was still some temp reading on the gauge.

I revved the engine a little and noticed the lights got brighter and idle got smoother. So I held the gas down for a few minutes, with the engine at about 1000 RPM, until the temperature gauge climbed considerably. Then I released the gas, and joy of joys - idle was once again at a silky smooth 650 RPM.

Here's my question: What does the ICV actually do? Does it just provide input to the ECU in order to create a fast idle until the vehicle warms up? I am thinking of cutting/interrupting the harness to the ICV valve and wiring a simple on/off toggle switch somewhere maybe under the dash or in the glove box. That way, I could activate the ICV when the car is cold, then deactivate it when it's no longer needed.

Does anyone have any thoughts or suggestions on this crazy idea? Has anyone tried it before? I'm guessing replacing the ICV is going to be a very expensive proposition, but I hate the thought of jimmy-rigging an otherwise perfectly functional vehicle.

Obviously, I could also just leave it the way it is, where it currently idles perfectly 33% of the time, idles way too high (~1,800 RPM) 33% of the time, and somewhere in between the other 33%. But I am concerned that I might be slamming my transmission into gear more than it was designed for. I currently take my foot off the brake right before the trans goes into gear whenever the idle is fast, in order to reduce stress on the trans. Can anyone comment on any potentially negative effects of occasionally putting the trans into gear at about 1,800 RPM? Also, when stopped at a light, am I heating up my torque convertor or causing some other form of damage to the trans itself? 1,800 RPM in N or P, about 1,000 RPM in gear.

I definitely know I am taking some life off the front brake pads...

Thanks in advance!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Observations/opinions anyone???

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Did you check the throttle position switch. (This is not the "microswitch" on the linkage.) The TPS is located at the end of the throttle valve, which is under the fuel distributor. The TPS has a built in pigtail to connect to the wiring harness and the connector is easily accessible above the inlet manifold on the front half of the engine.

The TPS has three leads - idle, WOT, and ground. You can easily test if it's working by disconnecting the pigtail connector and checking that it gives both the idle and WOT signals.

If it does not give the idle signal the car will not idle properly! A few years ago my idle got flaky - sometime okay, sometimes not, and sure enough a continutity check on the TPS showed that the idle signal was intermittent. Closing the throttle slowly would not provide the signal, but snapping it shut would. I just sprayed down the TPS with some mineral spirits to clean it, and then shot it will some WD-40 and it's worked fine ever since.

I definitely was NOT looking forward to changing it as the phillips head screws that secure it to the throttle body look just about impossible to access.

Modern cars have ICVs because a steady and stable idle is important to achieving emission standards, and it also make the idle speed "tamperproof".

Duke

Duke, thank you very much for the input. Can you explain a little bit more regarding the TPS? I don't quite understand exactly how it works. Are you saying it is an on/off type of component and should test as a closed circuit at idle? What should it read at WOT? Also, between which 2 leads should I be testing for continuity at idle? Would it be possible (or even safe) to unplug the pigtail while the car is idling high and jumper the 2 leads to see if that brings the idle down?

I will run out perhaps tonight and check that - I hope it's the culprit. But why did my idle improve so greatly when unplugging the ICV?

Oh, and any thoughts regarding my previous question of any damage caused to trans or torque converter as a result of elevated idle?

Again, thanks for your help everyone.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

One other thing, Duke - when you say you experienced a problem with your idle, do you mean it was idling high? I'm attempting to correct a high idle; it's not rough or anything, very smooth, just higher than it should be.

Thanks.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The TPS just has two sets of contacts - on or off - zero or infinite resistance - one for idle and the other for WOT. That's all it is!

IIRC the idle switch should be closed (zero resistance) with the throttle in the idle position. If it doesn't the ECU will not know that the throttle is in the idle position and will not command the ICV to maintain proper idle speed, so the idle speed will typically be high, which was the problem I had unless I snapped the throttle closed.

Trace the TPS pigtail to the connector on the top of the inlet manifold, disconnect it and check pairs to determine which is idle and which is WOT and check both for proper function. Do this with the engine OFF! The WOT switch tells the ECU to go into open loop mode and richen the mixture for maximum power. You can check resistance to ground on the engine harness side to determine which is ground - probably the center lead. Then check the pairs to determine which is idle and which is WOT. It shouldn't take much effort to figure would which pairs are which function. Exercise the throttle repeatedly to make sure each function is consistent. If the idle switch does not function or functions intermitently, snap it closed a few times to see if it works.

High idle speed will not damage the converter, but if you do a lot of stop and go driving it could cause the trans fluid to run at higher temperature, which will shorten the fluid's useable life, and it could also affect brake pad life.

Duke

Excellent explanation Duke - I think I can picture it now. I will try to look at it tonight, but if I get home late it will just have to wait until the weekend. In any event, I will report back what I find.

Thanks again for your helpfulness.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The ICV takes commands from the ECU. The ECU commands it to raise the idle when needed by opening a variable amount. Disconnecting the ICV closes it and you get the idle from the butterfly stops. As you found out, sometimes that isn't enough when the engine is cold.

Some sensor is telling the ECU that a higher idle is needed. My first guess would be the coolant temperature sensor is telling the ECU that the coolant is very cold, regardless of the actual coolant temp. It's fairly easy to check with an ohmmeter.

Best of luck,

__________________
Norm in NJ
Next oil change at 230,000miles

Like I said, the ECU will NOT command the ICV to maintain proper idle speed UNLESS it receives the signal from the TPS that the throttle is closed, so the TPS is the first thing I would check.

Duke

Thanks Duke, understood. I will check the TPS first. I am curious though, now that mention has been made of the coolant temperature sensor - does anyone know where it is and how to check it?

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

usually right next to the thermostat on the front of the engine, its usually a bolt like object with a two pin connector..


i lie - justy looked at the epc - its a bolt looking thing (either one or two pin) at the back of the head, near ish the injectors / inlet...

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