Low idling alongside stalling with a warm engine
Thread Starter
Newbie
Joined: Jan 2023
Posts: 4
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300E 1992
Low idling alongside stalling with a warm engine
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
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
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Joined: Mar 2016
Posts: 1,368
Likes: 195
From: SF Bay Area, CA
190E 2.6L 1989, 190E 2.6L 1988, BMW 535dXdrive 2014, BMW 428i 2015
Sure sounds like you just have an idle control issue from your description.
You have done quite a bit of diagnostics which helps us a lot.
Your IACV DC test results are perfectly normal, however you need to realize that there is a difference between how an IACV behaves with DC voltage versus a Pulse With Modulated AC voltage that averages equally to that DC voltage. All this being said I'm sure your IACV is just fine.
But to make sure measure the voltage on the IACV while it is idling at that low 300-400rpm and let us know what the value is.
Next unplug the IACV while the car is idling. Does the idle change to a higher value? Again let us know.
Messing with the duty cycle/lambda control just to fix an idle is indeed the absolute wrong thing to do. You have probably pushed the adjustment to a wrong setting now an will probably have to undo what you did with that and re-set it properly.
I would skip the fuel cleaners. Not necessary and these will hurt rather than help.
From the looks of it before you send out the results of the IACV tests I mention above this sounds like an idle controller problem and it is just sending or receiving the wrong value.
Perhaps the engine rpm is not received properly by the controller or....
(1) Have any parts of the ECU/MAS or ignition been changed with the wrong part number?
(2) What are the rpm's like in Neutral?
- Cheers!
You have done quite a bit of diagnostics which helps us a lot.
Your IACV DC test results are perfectly normal, however you need to realize that there is a difference between how an IACV behaves with DC voltage versus a Pulse With Modulated AC voltage that averages equally to that DC voltage. All this being said I'm sure your IACV is just fine.
But to make sure measure the voltage on the IACV while it is idling at that low 300-400rpm and let us know what the value is.
Next unplug the IACV while the car is idling. Does the idle change to a higher value? Again let us know.
Messing with the duty cycle/lambda control just to fix an idle is indeed the absolute wrong thing to do. You have probably pushed the adjustment to a wrong setting now an will probably have to undo what you did with that and re-set it properly.
I would skip the fuel cleaners. Not necessary and these will hurt rather than help.
From the looks of it before you send out the results of the IACV tests I mention above this sounds like an idle controller problem and it is just sending or receiving the wrong value.
Perhaps the engine rpm is not received properly by the controller or....
(1) Have any parts of the ECU/MAS or ignition been changed with the wrong part number?
(2) What are the rpm's like in Neutral?
- Cheers!
Last edited by dolucasi; Jan 19, 2023 at 11:46 PM.
Thread Starter
Newbie
Joined: Jan 2023
Posts: 4
Likes: 0
300E 1992
Sure sounds like you just have an idle control issue from your description.
You have done quite a bit of diagnostics which helps us a lot.
You IACV DC test results are perfectly normal, however you need to realize that there is a difference between how an IACV behaves with DC voltage versus a Pulse With Modulated AC voltage that averages equally to that DC voltage. All this being said I'm sure your IACV is just fine.
But to make sure measure the voltage on the IACV while it is idling at that low 300-400rpm and let us know what the value is.
Next unplug the IACV while the car is idling. Does the idle change to a higher value? Again let us know.
Messing with the duty cycle/lambda control just to fix an idle is indeed the absolute wrong thing to do. You have probably pushed the adjustment to a wrong setting now an will probably have to undo what you did with that an re-set it properly.
I would skip the fuel cleaners. Not necessary and these will hurt rather than help.
From the looks of it before you send out the results of the IACV tests I mention above this sounds like an idle controller problem and it is just sending or receiving the wrong value.
Perhaps the engine rpm is not received properly by the controller or....
(1) Have any parts of the ECU/MAS or ignition been changed with the wrong part number?
(2) What are the rpm's like in Neutral?
- Cheers!
You have done quite a bit of diagnostics which helps us a lot.
You IACV DC test results are perfectly normal, however you need to realize that there is a difference between how an IACV behaves with DC voltage versus a Pulse With Modulated AC voltage that averages equally to that DC voltage. All this being said I'm sure your IACV is just fine.
But to make sure measure the voltage on the IACV while it is idling at that low 300-400rpm and let us know what the value is.
Next unplug the IACV while the car is idling. Does the idle change to a higher value? Again let us know.
Messing with the duty cycle/lambda control just to fix an idle is indeed the absolute wrong thing to do. You have probably pushed the adjustment to a wrong setting now an will probably have to undo what you did with that an re-set it properly.
I would skip the fuel cleaners. Not necessary and these will hurt rather than help.
From the looks of it before you send out the results of the IACV tests I mention above this sounds like an idle controller problem and it is just sending or receiving the wrong value.
Perhaps the engine rpm is not received properly by the controller or....
(1) Have any parts of the ECU/MAS or ignition been changed with the wrong part number?
(2) What are the rpm's like in Neutral?
- Cheers!
