126 Idle Problems
 

I have a 1982 380 SEL. I have a fast idle problem. The engine runs fine and most of the time idles around 500 RPMs. However for no apparent reason, the idle will suddenly rise to 1500 to 2000 rpms. Then after a while it will drop back down to 500 then go back up after a few minutes.

I tried to kick down the trottle, no change. The thermostat has been changed, no change. I don't think its a throttle linkage problem as the idle will increase even when I don't touch the accelerator.

Any suggestions?

Thanks,
Brad.

These engines have a cold-idle/ idle control valve (ICV) that can fail or have issues, causing things like stalling or high/random idle. I'd check that out.

My wifes 190e did the same thing for months , then it progressed to engine stopping at random corners , then at every traffic light .

Both the Fuel pump and the OVP relays were pulled and resoldered , problems are all gone.

The v8s had a significantly different idle system from the early 4 and 6 cyl cars. The v8 idle controller's natural state is open. It has to be powered closed. It is powered by a variable duty cycle pulse. The more "pulse on" the more closed. It can be driven to 70% (70% is the highest duty cycle)closed. If it is 70% closed and idling too high false air (vacuum leaks) are occuring or the throttle isn't closed.

More commonly with symptoms such as yours the controller is deficient and releases control. This often is due to weak circuits occasionally repairable by resoldering.

The throttle has to close fully, and the throttle switch work consistnently. or the idle controller will not get a ground signal, and won't "know" that it should start controlling idle speed.

I fought with this on my 380SL, which the PO had allowed to sit for three years.

I started with a constant high idle. Found no voltage at the idle valve. Replaced the idle relay and got an intermittent high idle.

Cleaned the idle control valve, and it became less frequent.

Lubricated all linkage pivots, cleaned around the throttle stop, and cleaned the throttle body and throttle valve, (you will be amazed at what's there) and it became less frequent.

Replaced the throttle position switch, and cleaned the grounds a) at the right lower corner of the dash (you will need to check the location for the 126) and b) on the engine hoist point. Problem has not recurred.

So you have an interesting choice. It's almost impossible to tell if it's a throttle closing problem or the relay, if it's intermittent. So you can start chasing either depending on whether you want to spend money or tinker.

Chuck has a great list. I replaced my Idle control box '86 420SEL (passenger footwell) and still had occasional high idle problems; I took a clean cloth and WD-40 and cleaned the throttle body where the round air plate fits, and problems stopped. I also replaced the injector seals later, anyway. Then I found a vacuum leak in the climate control. Air leaks = idle problems. I dread replacing the rubber seals under the throttle body, but I know I'm going to have to sooner or later. There is a ground on the firewall, you can see it if you pull the instrument cluster. I did not check to see if it was accessible from below.
If you like puzzles and mystery novels and have an alternative vehicle, you'll make a great Mercedes-Benz hobbyist.

It sounds like the idle control valve is working. Check the idle control module.

Thanks for all of the suggestion and the "list."

I will see how difficult it is to take out the ICV. How would I check to see if its working or better yet, clean it?

Thanks...

Find a plug to seal one of the holes. Squirt injection cleaner in the other. Shake it and set it up so the cleaner doesn't run out. Let it sit for four hours, shaking it a couple of times. Dump the cleaner out. Repeat until the fluid comes out clean.

Thanks Chuck:

I've found replacement valves between $200 - $800! I'll try cleaning it....