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  #1  
Old 06-27-2003, 04:28 PM
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Join Date: Feb 2002
Location: Philippines
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M104 fluctuating idle speed

Gents, My 95 C36 suddenly developed a peculiar idle today. It fluctuates from 1000 to 1700 RPM in approx 1-second cycles, in gear or in neutral. This is scary because the car speeds up by itself when the RPM climbs and brakes can hardly hold it.

I shut the motor off but the problem was still there when I re-started. I shut it down once again and when I fired it up after a couple of seconds, the idle was now steady but very low (400-450 RPM). There are no check engine lights showing and everything else seems normal.

I did a search but can't find any posts with these particular symptoms. The plugs are about 1 year old (less than 15,000 km) and car is very smooth and powerful until today.

Assistance in troubleshooting this malady is highly appreciated.
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  #2  
Old 06-27-2003, 08:12 PM
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Join Date: May 2003
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BongC36,

I've searched for posts regarding the same problem on a 300E 3.2 and someone mentioned the mass air sensor as the culprit. That kind of surprised me but I pass it on as a point of information. You might consider checking the sensor and connections.

My first thought was the "electronic accelerator pedal" and its wiring harness since that's what controls idle speed. I think the '95 C class cars still suffered from wiring harness problems so it might be worth jiggling that harness to see if the symptoms change any.

Since the 104 engine was used in 124 cars from '93 on you could search for idle problems on 300Es in hopes of yielding more search results.

The problem I was chasing went away during the course of a lot of other unrelated work but the only part I replaced that might have affected the idle was the camshaft position sender.

Good luck and let us know what you find.

///MikeR
'93 190 2.6 Sportline
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  #3  
Old 06-27-2003, 08:25 PM
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You may have read my post on the fluctuating idle. My fluctuating idle was exactly what you are describing.

My problem was a vacum leak - at the intake manifold - which is quite an unusual place for a vacum leak.

A vacum leak (anywhere in the system) is "false air", and prevents your mass air meter from figuring out the proper air / fuel mixture. The idle will hunt up and down, trying to find the right mixture, and can't because of the vacum leak, and keeps hunting.

If you don't have a vacum leak, then your mass air meter may simply be defective.

Additionaly, a defective throttle actuator will exhibit similar symptoms. The throttle actuators have pentiometers that go bad over time. They also have a wiring harness that goes bad - just like the engine wiring harnesses that fail in all M104 engines from 1993-1995.

At any rate, there are too many variables: take it to the dealer or an independent mechanic that specializes in Mercedes Benz vehicles.
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2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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  #4  
Old 06-27-2003, 10:01 PM
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Location: Toronto, Ontario
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It sounds like a vaccum leak to me as well. Did you change the oil filter recently? My dealer knocked a vaccum line off at an oil change once and my idle started doing strange things as well. It was when the tech removed the filter.

Jonathan
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  #5  
Old 06-28-2003, 05:41 AM
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Thank you all for the prompt replies.

I am beginning to suspect the MAS or an intake leak too, because we did remove the cross-over pipe (from the air cleaner to the intake manifold) to get at the plugs the other day.

Where did your leak occur Suginami? The two ports at the bottom of the cross-over piece seems deformed on mine. I re-inserted snugly it and now it seems ok but time will tell. BTW How do I test a MAS without a diagnostic computer? I am quite far from the dealer...
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  #6  
Old 06-28-2003, 08:06 PM
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Quote:
Originally posted by suginami

My problem was a vacum leak - at the intake manifold - which is quite an unusual place for a vacum leak.
The leak was at the intake manifold gasket.

I have read posts from the technicians that help out on this site, that they usually test for a bad MAS by swapping it with a new one.

It takes more time to diagnose one that it does to replace it. They only take about 5-10 minutes.
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2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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  #7  
Old 06-28-2003, 08:25 PM
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< >>

After cross pipe removal, it is most common to find a leak at the cross pipe clamp end opposite the MAF end.
This clamp reqiures a Loooong screwdriver to tighten and needs to be very secured...
Have also seen AIT sensor not plugged /seated back in.....
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  #8  
Old 06-29-2003, 05:28 AM
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I was able to get about a quarter turn more when I re-tightened up the clamp at the intake end. Symptoms have not yet reappeared since so hope that that is it.

Thanks for the responses guys.
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