Low idling alongside stalling with a warm engine

[Berinchtein]

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

 

[rayhennig]

I think you have a lot of research ahead of you if you take this car seriously. Briefly, you need to make sure that:
> There are no vacuum leaks.
> The HT system is 100% perfect, from coil to plugs - these cars demand it.
> The air intake flap is smooth in operation.
> The mixture screw has not been tweaked by people who don't know how. Measure the Duty Cycle at Pins 2 & 3 on the little round 9 Pin diagnostic socket. Search the net on this, particularly a guy in Germany called H.D.

Good hunting.

RayH

 

[DREAMER NO2]

Ray in his post above knows the drill with these W124 s .. Check all vacuum hoses, and small bore pipes , for any damage .If you find a split ,,or hose missing then thats the problem. And i tell every one to make sure HT system is up to 100% . With the right spark plugs fitted . And any plug with R in the number is taboo only use none resistor spark plugs .. Even battery conections, lose or dirty could hinder how it starts and runs . And corroded fuses will cause havoc . ..never ever move the mixture screw in the mettering unit on top of fuel distributor its the last item in the list to look in to ,but you have . Air filters and fuel filters will need changing for piece of mind .

 

[Berinchtein]

Hi, this is an important follow-up to this post: https://forums.mercedesclub.org.uk/...alongside-stalling-with-a-warm-engine.196342/

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.

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.

 

[Berinchtein]

I think you have a lot of research ahead of you if you take this car seriously. Briefly, you need to make sure that:
> There are no vacuum leaks.
> The HT system is 100% perfect, from coil to plugs - these cars demand it.
> The air intake flap is smooth in operation.
> The mixture screw has not been tweaked by people who don't know how. Measure the Duty Cycle at Pins 2 & 3 on the little round 9 Pin diagnostic socket. Search the net on this, particularly a guy in Germany called H.D.

Good hunting.

RayH

I checked the last two ones but I'm unsure about the first two things to make sure. I'll see hoe I can assure that they're fine, especially the vacuum leaks

 

[Berinchtein]

Sorry, I should've updated my old thread instead of posting a new one. Don't hesitate to delete this one

 

[Blobcat]

Sorry, I should've updated my old thread instead of posting a new one. Don't hesitate to delete this one

I’ve merged the 2 threads for you

 

[rayhennig]

red on pin 3 and the black on pin 4

It's Pin 2 and Pin 3. You should see the duty cycle varying, say from 46 to 54 or something. But you imply it's stuck at around 50. When the engine has heated up and the system has entered "closed loop" the fuel delivery is changed according to what the O2 sensor reports from the exhaust.

It's important to understand this concept before trying to fix things.

Regarding the HT system, have you checked the distributor? Is it clean? What do the spark plugs look like? And what part number on the plugs? New, non-resistor plugs are not expensive from MB.

RayH

 

[mattkh]

Sorry,

Some electrical things misbehave when they get warm.
Do you know what generates the high voltage for the spark plugs on your car engine..????

 

[DREAMER NO2]

Poor main coil . Run engine in tyhe dark and look for flashing , but pull the cover off the coil first
Next take a look at your fuel tank filler cap seal,, is it in good condition ?

 

[Berinchtein]

I’ve merged the 2 threads for you

Appreciate it!

 

[Berinchtein]

It's Pin 2 and Pin 3. You should see the duty cycle varying, say from 46 to 54 or something. But you imply it's stuck at around 50. When the engine has heated up and the system has entered "closed loop" the fuel delivery is changed according to what the O2 sensor reports from the exhaust.

It's important to understand this concept before trying to fix things.

Regarding the HT system, have you checked the distributor? Is it clean? What do the spark plugs look like? And what part number on the plugs? New, non-resistor plugs are not expensive from MB.

RayH

Oops my bad. I'll try this next time. It was indeed weird how the duty cycle wouldn't indicate anything else besides 50.25%. The engine must have been heated up a little, because I tried checking something else a couple of minutes before. The distributor is something I forgot about. I'll have a look at it and check for some deposits. I changed the spark plugs with something that would indicate being compatible with my engine, just in case. The manufacturer is Champion and the model is Copper Plus. The part number would be 401 RS12YC

 

[rayhennig]

401 RS12YC

Not ideal. That's a resistor plug and not recommended by MB. Is there not a label near your engine that states the plug reference? On mine, the label gives Bosch, Beru and Champion references.

Regarding Duty Cycle (this is for my car, yours may be different), it should read 70 when cold and 50 when hot, ignition on engine off. When hot and engine running, varying around 50, as said.

R

 

[Berinchtein]

Not ideal. That's a resistor plug and not recommended by MB. Is there not a label near your engine that states the plug reference? On mine, the label gives Bosch, Beru and Champion references.

Regarding Duty Cycle (this is for my car, yours may be different), it should read 70 when cold and 50 when hot, ignition on engine off. When hot and engine running, varying around 50, as said.

R

Oh yeah, it was at 70 when the ignition was on at least.

 

[rayhennig]

it was at 70 when the ignition was on at least.

It sounds as if your ECU is OK.

R

 

[mioba]

Ovp problems are all the time, you have a a vac leak by sounds - gets worse when engine hot (things expand).
Great engine to work on - lots of space.

 

[Berinchtein]

Ovp problems are all the time, you have a a vac leak by sounds - gets worse when engine hot (things expand).
Great engine to work on - lots of space.

Indeed. It at least feels good to work on.

For the vacuum leaks, you really enlightened me on something here. A lot of people have been telling me that, but I was too unprepared to do it properly. The fact that it expands when hot makes a lot of sense. I'll check for real

 

[DREAMER NO2]

So tell us what spark plugs are in your engine ???? might not be the right ones .

 

[DREAMER NO2]

For others .

 

[mioba]

Indeed. It at least feels good to work on.

For the vacuum leaks, you really enlightened me on something here. A lot of people have been telling me that, but I was too unprepared to do it properly. The fact that it expands when hot makes a lot of sense. I'll check for real

Its phenomenal how much is vac controlled on a W124. great engineering