Low idling alongside stalling with a warm engine

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Berinchtein

New Member
Joined
Jan 17, 2023
Messages
10
Location
Montréal
Car
Mercedes-Benz 300E 1992
Hi, I bought a 250k km 1992 300E 2 days ago as my first car and project. It drives pretty well, but it has some issues. The major one is that the engine idles at a very low RPM (~300-400) after driving the car for some time. I've read many forums and watched many YouTube videos, with little help. Here are the details:

The Problem:

The engine fires up very quickly. When first starting it after some time since the last use, it idles at around 1000-1100 RPM. It feels really smooth and drives nicely. After a while though, the RPM slowly drops until it reaches around 300-400 RPM. This makes the car very shaky and makes it sound like a dragster. When depressing the pedal in a forward gear, the car launches agressively as it jumps from the low RPM to a more stable one required for normal driving circumstances. Here's how each gear acts in the idle state:

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)
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

What we've tried:

The first thing I learned about was the OVP relay. I decided to unplug it 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 it, the engine worked as usual.

The second thing was the Idle Control Valve (ICV). I saw some people changing/cleaning it 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. We didn't clean the valve by lack of the required product but it seemed clean.

The third is the fuel mixture adjustment. I read some owners on the forums who fixed their problem by leaning the mixture. I turned the knob with the required Allen key but didn't notice any difference. I was a bit fed up and rebuilt the air filter to go for a drive, which was no different from the ones before the first 3 attempted diagnostics.

The fourth thing I'm gonna do is add some fuel system cleaner the next time I fuel up. This might be in a while, because the car (fortunatly) doesn't burn fast. The fuel tank is currently half-full with 89 Octane gas because of a mistake I made, and I hope the combination of a high octane gas & system cleaner will help. Here's the product, just in case: General PDP Template

Now

I'm wondering what I should do. If anyone has an idea of the next step I should be taking to solve my problem, please let me know. I'm very new to the mechanics world and do not have a very extended knowledge. Any guide or tutorial linked with the given advice would be truly appreciated. Feel free to ask any question, I'll be here to answer them.

Have a nice day,

Manuel
 
Have you cleaned inside the throttle body? They often get black with a build up of gunge and restrict the amount of air getting into the intake, particularly at idle. I’d start with that
 
Have you cleaned inside the throttle body? They often get black with a build up of gunge and restrict the amount of air getting into the intake, particularly at idle. I’d start with that
Not yet, I'm buying a Throttle Body Cleanner tomorrow. Thanks!
 
I assume this a Bosch Ke injection although by 92 I expect it had a cat and closed loop control so it will be a little different from the pre cat set up I'm familiar.

Does it not have a idle speed adjustment like the earlier Bosch Ke set up. It's that big screw visible to one side of the rocker cover under the air filter. If it has, it's just acts as simple air bypass which will increase idle speed if turned anti clock wise. There was a tendency for this to get gummed up by oil from the engine breather which would reduce the idle speed. What I used to do was turn it in gently as far as it would go to count the number of turns as an aid to putting it back at the same setting. Then turn it anticlockwise and remove it altogether. You could then see if things needing cleaning up, mine always did.
 
I assume this a Bosch Ke injection although by 92 I expect it had a cat and closed loop control so it will be a little different from the pre cat set up I'm familiar.

Does it not have a idle speed adjustment like the earlier Bosch Ke set up. It's that big screw visible to one side of the rocker cover under the air filter. If it has, it's just acts as simple air bypass which will increase idle speed if turned anti clock wise. There was a tendency for this to get gummed up by oil from the engine breather which would reduce the idle speed. What I used to do was turn it in gently as far as it would go to count the number of turns as an aid to putting it back at the same setting. Then turn it anticlockwise and remove it altogether. You could then see if things needing cleaning up, mine always did.
I'll take a look, although I may have trouble finding it.
 
Did you have the problem before you put th 89 octane in. If not you may have answered your own question.
 
Did you have the problem before you put th 89 octane in. If not you may have answered your own question.
Unfortunatly I don't know. I bought the car a couple of days ago and only had to refuel it right after the purchase, where I did the mistake. I still have not consumed this tank, so I'll have to check once I go with 91/95 Octane.
 
Unfortunatly I don't know. I bought the car a couple of days ago and only had to refuel it right after the purchase, where I did the mistake. I still have not consumed this tank, so I'll have to check once I go with 91/95 Octane.
Octane is for detonation prevention and detonation never occurs during idling. I doubt very much that the lower octane fuel is the cause of your idling problem.

Mixture strength will impact idle quality though. I'd pull a spark plug when it is playing up and see if it looks like it's running rich or weak and then use that information to pursue the problem (eg, little point in chasing (non-existent) vacuum leaks if it's running rich) and, as the problem worsens as the engine warms, an overly rich mixture is a prime suspect.
 
He's in Canada...89 octane there is the same as normal 95 octane here....
 
The problem has evolved a bit since I did some cleaning. Here's how it goes now:

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.

Someone recommended me to buy a new OVP relay 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 cleaned the Idle Control Valve (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.
 
Octane is for detonation prevention and detonation never occurs during idling. I doubt very much that the lower octane fuel is the cause of your idling problem.

Mixture strength will impact idle quality though. I'd pull a spark plug when it is playing up and see if it looks like it's running rich or weak and then use that information to pursue the problem (eg, little point in chasing (non-existent) vacuum leaks if it's running rich) and, as the problem worsens as the engine warms, an overly rich mixture is a prime suspect.
I'll see how to check that. Thanks a lot
 
The problem has evolved a bit since I did some cleaning. Here's how it goes now:

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.
What is the temperature and humidity in Montreal? The above sounds a lot like carburettor icing - except there's no carb.
(There is I think the possibility of ice build up adversely affecting fuel metering on a Bosch K system - but only if the ambient temp and humidity are conducive to ice formation).
 
What is the temperature and humidity in Montreal? The above sounds a lot like carburettor icing - except there's no carb.
(There is I think the possibility of ice build up adversely affecting fuel metering on a Bosch K system - but only if the ambient temp and humidity are conducive to ice formation)
There was around three days during which I worked on the car in temperatures around -7 degrees celcius. Yesterday, the temperature was more around 0 degrees. This may have a link with the change in behaviour. For the humidity, my weather app indicated an average of 80% these days
 
There was around three days during which I worked on the car in temperatures around -7 degrees celcius. Yesterday, the temperature was more around 0 degrees. This may have a link with the change in behaviour. For the humidity, my weather app indicated an average of 80% these days
Google '' Bosch K icing up'' and see what can be found. If anyone here has experienced icing in a Bosch K system - please post.
I've experienced carb icing in similar temps so the conditions are conducive. I can elaborate (later) on how I think Bosch K could be so affected but finding that others have experienced it would be a step forward I think.
 
One way to check for vacuum leaks is with the engine running spray carb cleaner on the pipes, one at a time, if you can get access to them. If there is a leak/draw the engine revs will increase.
 
One way to check for vacuum leaks is with the engine running spray carb cleaner on the pipes, one at a time, if you can get access to them. If there is a leak/draw the engine revs will increase.
Good to know. I tried this on some of the hoses but I wasn't sure it worked. I'll try again, thanks!
 
Does it not have a idle speed adjustment like the earlier Bosch Ke set up.
Unless I misunderstand your description, there is no idle speed adjustment. Idle speed is controlled by the Ke system. There is an adjustment that chqnges the fuel delivery duty cycle but that is not something to twiddle without understanding the implications.

Search on Google "ke jetronic duty cycle" or similar.
 

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