Low idling with stalling after some time

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New Member
Jan 17, 2023
Mercedes-Benz 300E 1992
Hi, this is an important follow-up to this post: Low idling alongside stalling with a warm engine | Engine

My situation has improved a bit but the problem still brings me many issues. Even though I'm still motivated to fix the issue, I'm becoming tired and not sure what to try anymore.

The Problem:

When first starting the engine after some time since the last use, it idles at around 750 RPM. It feels really smooth and drives nicely. The RPM will then slowly drop until it reaches around 400 RPM. That would be after around 45 minutes of driving. Under 500 RPM, the car becomes very shaky and sounds like a dragster, pressing on the car pedal makes the car launch in a very sharp and uncomfortable way, the rev-counter needle indicates a rough/jumpy idle and the cabin starts smelling like gas very clearly. I'm not sure it is linked to the problem but it unfortunatly probably is. Here's how each gear acts in this situation:

P: The engine stalls after a couple seconds when the RPM is at its lowest (~300)
R: The engine stalls each time there's a small inconvenience in the way I drive in reverse (hitting some snow, braking too firmly, turning too sharply). This makes parking in parallel a huge issue.
N: Same as P
D: The engine doesn't let itself stall. This gear works best, even with the aggressive launch
3: No idea
2: The engine stalls right after I release the pedal when the RPM is at its lowest (~300)

What I've tried:
(I underlined the new parts added since the first thread)

I decided to unplug the OVP relay and have a look at it. Everything looked good from the exterior, but I wasn't able to remove the relay from its metal casing. The other models I've seen in the YouTube videos had handles that you could separate to remove it, but the one I have was built by Mercedes and dates the 30th of Febuary 1992. It doesn't seem to have been made for being taken apart. When starting the engine without it, the RPM stayed at a low ~500 RPM and pressing on the throttle did nothing. When plugging it back in, the engine worked as usual. Someone recommended me to buy a new one even though I'm not sure if it's the problem. I shopped for one but couldn't find an affordable genuine Mercedes one. I'll have to buy an aftermarket (URO parts/Vemo) version once I match the numbers.

I saw some people changing/cleaning the Idle Control Valve (ICV) to fix my problem. I removed it and we did some tests. First, we put some voltage on the pins. The valve was closed at around 3V and stayed open below and above this voltage. After that, we checked the voltage coming into the ICV from the car with the cables plugging into it, which was around 2V. We plugged the ICV back in place and put a voltage source on its pins. The engine wouldn't start at 0V but started quickly with 2V. Increasing the voltage increased the idle RPM until about 1200. I cleaned the ICV & air intake flap with a Throttle Body Cleaner today, which seemed to change my situation a little bit. I wasn't able to reach the Throttle Body itself though, as it is installed below the fuel distributor / air intake module. Trying to unscrew some hoses from the module caused fuel to pour out so I stopped. I'll try again with more precautions next time.

For the fuel mixture, I'm puzzled. I was stupid by trying to turn the mixture adjustment screw myself, but it doesn't seem to have changed much. I checked the Duty Cycle the next day by plugging the red on pin 3 and the black on pin 4. It was at 50,25% and I decided to leave it like that.

The most worrying and uncomfortable thing to check for me is the vacuum hoses. I tried to spray some with the cleaner to try and notice any change in the running engine behaviour, but nothing changed. This was the way I saw some people do it on forums/YouTube videos but it might be wrong. Please tell me if you have any advice for checking vacuum leaks, I have no idea how else to do it without messing something up.

My 89 Octane tank (a mistake) is almost empty, so I'll fill it up with 91/95 soon. If I'm lucky, the issue came from here. It's very unlikely but I'm hoping for some miracles right now.


As it might have been noticed, I seem to have done something which changed the way the idle works, but it has not improved my situation a lot. I wanted to update people in case they still want to help a little.

All the given advices have been very appreciated and I wanted to thank the community. I'll make it thanks to you guys.
Sorry, I should've updated my old thread instead of posting a new one. Don't hesitate to delete this one

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