M104 idles low when cold - Mercedes-Benz Forum
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post #1 of 15 (permalink) Old 11-21-2016, 08:25 AM Thread Starter
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M104 idles low when cold

M104.992 on 1995 E320C (euro) W/O EGR, CC, ASR used to:
-start very quick every time (winter, cold engine), go up to ~1300rpm then ~1100rpm wait there for a while and when engine warmed up drop slowly to warm idle speed without ever stalling or misfiring.
-when driving, and manual transmission (dogleg) was put to neutral and the car was coasting rpm used to drop to ~1300rpm, stay there for a second or two and then slowly drop to idle speed, never misfire or stall; same when changing gears, the drop in rpm was never abrupt;

It developped following habit:
-starts quick with all 6cylinders, runs "straight" (winter, cold engine) but rpm never go up, it remains at ~500-600; if I accelerate sometimes stalls. If I let it be for a minute I can drive the car just fine, it is responsive, pulls like a freight train and everything appears to be just fine;
-rpm drops very fast do warm idle speed between gear changes;
-engine doesn't compensate by going rpm up when AC is ON; at a red light when AC clicks/turns on idle drops to ~500 and sometimes engine stalls; AC goes off - idle speed goes back up a little.

Engine wiring is replaced;
Due to AC on/off I suspect engine coolant temperature sensor doesn't tell ECU engine temperature (the one with 4 pins) but I was wrong many many times before so...

feel free to share if you have an ideea.
P.S. No, I wasn't able to find anything usefull using "search" function...
Regards,
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post #2 of 15 (permalink) Old 11-21-2016, 10:56 AM
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First step is determining if your car is suffering from the eco junk wiring bug.

Without a good engine harness and throttle body, made in 1998 or later, these cars cannot be properly diagnosed.

PeachPartsWiki: W124 Engine Wiring Harness Replacement
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post #3 of 15 (permalink) Old 11-21-2016, 11:36 AM Thread Starter
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Engine wiring was replaced by previous owner and everything looks fine. I don't know about the throttle boddy though.
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post #4 of 15 (permalink) Old 11-21-2016, 01:41 PM
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was the throttle body / idle speed controller rewired or replaced, too? they also have the eco-junk insulation, and could easily contribute to those symptoms. in the US there's two versions, one with cruise control and idle control, and the other with CC/ICS and electronic throttle, used for ASR. I don't know what you'd have if you don't have CC (maybe they use the CC/ICS version and just don't connect the CC part?). there's a date code on the label of hte throttle body in the format 94M08 (8th month of 1994). this has to be 1998+ to be good. there's green label and grey(white?) label throttles, the green ones are CC/ICS, while the white ones are EA/CC/ICS and are used on cars with ASR, they are not interchangable. ALWAYS replace the gasket when removing the throttle body.


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post #5 of 15 (permalink) Old 11-21-2016, 10:11 PM Thread Starter
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Aha, I will check manufacturing date and come back; this actuator seems to be "easy" to rewire, if necessary. Unfortunatelly replacement with a new unit is not an option.
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post #6 of 15 (permalink) Old 11-21-2016, 10:24 PM
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the hard part of doing your own rewiring is access to the right grade of wiring with the right color codes. otherwise you end up with a bundle of undocumented ugh, and make life miserable on yourself or the next guy who has to look at it.

be sure to use the correct gauges of automobile grade oil resistant wire as its a filthy hot environment in the engine compartment. CC/ICS has like 11 pins connected, using a mix of 0.5, 0.75 and 1.0 mm^2 wiring. Inside the CC/ICS, there's a motor, a solenoid, and 2 potentiometers, along with some limit switches.


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post #7 of 15 (permalink) Old 11-22-2016, 12:16 AM Thread Starter
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I understand.
blue labeled 95M06 so I gess it's OEM (car is 1995).
Later today I will start the engine wiggle things a little and see if there is a change. I will report back.
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post #8 of 15 (permalink) Old 11-22-2016, 12:32 AM
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can you read the mercedes part number on it? that would be a XXX XXX XX XX number immediately after a mercedes logo. the CC/ICS green label module is 000 141 57 25, like this...



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post #9 of 15 (permalink) Old 11-22-2016, 12:51 AM Thread Starter
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Here:
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post #10 of 15 (permalink) Old 11-22-2016, 06:47 AM
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My wife's car (1993 300CE w/M104.992) had this same cold-idle stumble as well as a non-functioning cruise control. I replaced the throttle assembly with a unit built in 2001 and all the symptoms disappeared and the CC now works. I found the unit on eBay for $399 and bargained it to $325 with free shipping.

Edit: BTW- Your post is the first I have seen with a symptom exactly like mine, and I looked extensively. I was going to refrain from asking the forum members until I eliminated the eco-junk components... now I don't have to ask!

Last edited by Earl.Allen; 11-22-2016 at 06:50 AM. Reason: additional stuff
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