Intermittent Stall + High Idle - Mercedes-Benz Forum
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post #1 of 79 (permalink) Old 01-06-2019, 01:10 PM Thread Starter
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Intermittent Stall + High Idle

Having an issue with my 2.6. I keep running into a rare stalling while stopping situation that comes back just as I think it's been fixed. Basically the car will shut down when I start to slow down, noticed it coming to lights, pulling into parking lots etc. The car always immediately restarts, has never not started right back up, however I'm also chasing a slightly hard start situation as well so sometimes it needs some gas. The last time it happened I noticed that idle in Park and Neutral jumped to around 1500rpm before and after the stall. After I let it sit and then restarted all was good and idle was normal. The stalls started when the car was under 1/4 tank so I replaced the fuel filter and then found a pump leak and replaced both pumps. Thought it was fixed for awhile but now its happening again with above half a tank, can't seem to pinpoint any commonalities when it happens.

Any thoughts?
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post #2 of 79 (permalink) Old 01-06-2019, 01:29 PM
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Number one suspect is OVP, number two would be the microswitch.

I had the exact same symptom with my '89 for over a year. After 4 months of diagnosing etc. it turned out to be the OVP.
Especially if it is racing to 1500rpm prior to stall, that is a classic symptom.

When OVP cuts off, the engine goes to limp mode momentarily and forced a higher idle (like 1500rpm) and when the OVP comes back on seconds later, the IACV overreacts and the engine stalls.

Check the date code on the OVP, if it is 10 years or older replace it with OEM or KYE. You can not test an intermittent failure like this so repair is futile.

At the same time make sure the microswitch is operating properly.

Also make sure the OVP terminals are super clean.

I doubt the issue is related to the fuel level in your tank. Most likely coincidence.

I diagnosed this issue for 4 months before we converged on the real culprit (this all happened on the W124 side).

-Cheers!
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post #3 of 79 (permalink) Old 01-06-2019, 01:37 PM Thread Starter
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OVP was the first thing I replaced when I bought the car, with a KAE unit. Only thing I found was the plastic mounting bracket the OVP slips in to was not actually attached to anything and the wires are quite short, it took some pulling and prying just to remove the OVP. There is a chance that some of the wiring was broken when either I or the PO installed one, the old one was replaced about 5 years ago. That said, no idea how to test if it’s operating correctly or not, there’s just no way to get to anything under there.
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post #4 of 79 (permalink) Old 01-06-2019, 02:55 PM
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Quote:
Originally Posted by dolucasi View Post
I diagnosed this issue for 4 months before we converged on the real culprit (this all happened on the W124 side).

It took 4 whole months, with your buddy helping you?

Sigh....

Thereís a way to test the microswitch. I think you raise the RPM to 2000 or 2500 and then manually press the wheel so it activates the little button. Engine should hunt.

Kind of busy to find it right now. Son has Science Olympiad competition in two weeks and was just asked to add Meteorology. Heís got a lot of material to cover, which includes some stuff that boggles my mind.......
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post #5 of 79 (permalink) Old 01-06-2019, 03:26 PM
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Quote:
Originally Posted by maranelloboy05 View Post
OVP was the first thing I replaced when I bought the car, with a KAE unit. Only thing I found was the plastic mounting bracket the OVP slips in to was not actually attached to anything and the wires are quite short, it took some pulling and prying just to remove the OVP. There is a chance that some of the wiring was broken when either I or the PO installed one, the old one was replaced about 5 years ago. That said, no idea how to test if itís operating correctly or not, thereís just no way to get to anything under there.
From your description, your wires/connectors would be suspect. To diagnose this, you can stick two wires into the IACV connector and pull them thru the firewall into the car, and monitor the IACV voltage all the time. Mine was happening once every 3-4 days and hard to capture but if you notice the IACV voltage going to zero volts and then coming back on right before the stall before coming to a stop that would be the sign.

You can also check continuity between your OVP and the ECU and move around the wires to see if you can detect intermittent connection.

Microswitches can also go bad but they have a really long life. Mine is original at 190K miles. And it being a manual transmission, that microswitch has been exercised many multiples times over a auto transmission car. My car is primarily driven in city, and everytime I change gears that microswitch if flipped off and on again. We must be at 50K cycles and counting.

But of course you should test yours to make sure.

- Cheers!
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post #6 of 79 (permalink) Old 01-06-2019, 03:31 PM
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Originally Posted by jzchen View Post
It took 4 whole months, with your buddy helping you?

Sigh....

Thereís a way to test the microswitch. I think you raise the RPM to 2000 or 2500 and then manually press the wheel so it activates the little button. Engine should hunt.

Kind of busy to find it right now. Son has Science Olympiad competition in two weeks and was just asked to add Meteorology. Heís got a lot of material to cover, which includes some stuff that boggles my mind.......
Well it sounds like we have you beat by a year or more, you are still trying to figure out what your problem(s) is(are) JZ ......
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post #7 of 79 (permalink) Old 01-06-2019, 06:27 PM
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Well it sounds like we have you beat by a year or more, you are still trying to figure out what your problem(s) is(are) JZ ......
I saw this device, that your friend made, that could have solved your problem in what he claims as less than a minute:

https://www.benzworld.org/forums/w12...st-device.html

I finished the air flow pot, besides the initial stall it has not stalled since. I drove it maybe 5 or 6 times. It is back on a battery maintainer as a back up car like it has been, in my book problem solved as it no longer has a check engine light with the original pot patched. I posted my methodology on that thread and I have moved on to looking at my friend's W212....
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post #8 of 79 (permalink) Old 01-06-2019, 06:56 PM
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Originally Posted by jzchen View Post
I saw this device, that your friend made, that could have solved your problem in what he claims as less than a minute:

https://www.benzworld.org/forums/w12...st-device.html

I finished the air flow pot, besides the initial stall it has not stalled since. I drove it maybe 5 or 6 times. It is back on a battery maintainer as a back up car like it has been, in my book problem solved as it no longer has a check engine light with the original pot patched. I posted my methodology on that thread and I have moved on to looking at my friend's W212....
Good to hear. Drive it for 2 straight weeks before you claim victory....I had gone long periods like 2-3 weeks of driving before an unexpected stall returned coming to a stop or right in the middle of down shifting. I sincerely hope it was your AFM pot but very much doubt it .....

HD has posted a picture of that in a much earlier thread and If my memory serves me right, I was one of the first members who asked him to send info on that gizmo 2-3 years back. That is priceless, however what I have instead is what I suggested the OP here. A couple of long wires pulled thru the firewall and can reach anywhere in the engine.
It is coiled behind the battery at the moment as it has not been used in a year and a half. First sign of any issue, I'll connect it to the relevant sensor/actuator and observe it while driving. It is your poor-man's HD tool. Would recommend it to anyone and everyone with a M102/M103 engine.

Since you are paranoid about the duty cycle, if you had these wires you could watch the duty cycle of the lambda all they long while driving.

-Cheers
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post #9 of 79 (permalink) Old 01-06-2019, 10:35 PM
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It would be great to have an electrical diagram for the OP to trace and confirm the wiring for the ovp.

This IACV thing seems interesting. Worth a try. So stick wires into the connector and read the voltage; also it will be helpful to know what the voltage should be and how it should behave.


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post #10 of 79 (permalink) Old 01-06-2019, 11:43 PM
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Originally Posted by noetico View Post
It would be great to have an electrical diagram for the OP to trace and confirm the wiring for the ovp.

This IACV thing seems interesting. Worth a try. So stick wires into the connector and read the voltage; also it will be helpful to know what the voltage should be and how it should behave.


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I do not have a circuit diagram PDF handy but here are the critical connections from my Haynes Manual:

OVP - Pin 30 to Battery positive terminal
OVP - Pin 31 to Chassis ground
OVP - Pin 15 to FPR pin 15
OVP - Pin 87 to ECU pin 1

Probably should also check the IACV connections to the ECU.

As for the average voltage on IACV, each car will be a little different because mechanically they are not created equal and also depends on system voltage. At idle they will start around 5V (cold) and drift down to around 4V average.
When the idle jumps to 1500 rpm confirm that IACV is zero (most likely OVP has cut off)
Right before stall, see if the IACV voltage jumps from zero to a positive value (most likely the OVP just woke up)

You can emulate this very malfunction by unplugging the IACV during idle and observe the idle increase to over 1000rpm. Then during idle plug it back in and observe the immediate stall.
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