Slight surging at idle in gear, S124/M111

[OneCarefulOwner]

My S124 is suffering from an intermittent surging at idle when in D/3/2/R; sometimes it'll settle nicely on 750rpm, other times it'll pump from 500 to 1200 and back, or higher if I put the shifter in N/P. When it's surging, sometimes revving past 3k will fix it, other times it'll keep doing it. Usually it won't affect driving, but if I'm doing some low speed manoeuvres, into a parking space for example, it'll very rarely stall out - only twice has this ever happened.

Should I check the engine bay wiring loom for degradation as mentioned in the sticky, could it be a vacuum/air leak, or is there any other suggestions? I have my own spanners, a Haynes manual, and a pet (non-MB) mechanic if it gets really technical.

 

[Beamish]

Hi,

The M111 engine was not affected by the harness problem. I imagine that your problem is related to your throttle body. Perhaps you could try giving it a thorough cleaning using throttbe body or carb cleaner.

Beamish.

 

[trapperjohn]

How many miles on your E200? My ex 124 E220 Estate suffered "surging" and a "younger" wiring harness sorted this fault. Milage at the time was 277,000+ Sold the car on and as far as I am aware it is still running fine and heading for 300,000 miles and is in daily use.

 

[Ian B Walker]

This could be a number of things. Personally I would look at the throttle body, it may need cleaning or bootstrapping. Or as per above, could be the wiring loom although I seriously doubt it.

 

[OneCarefulOwner]

It's a youngster with a mere 215k miles on it.

I went out in the car twice yesterday after posting, and the first time this didn't happen. The second time, it did and then didn't and then did again... And I've kind of figured it out: it was happening when I came to rest with a high load on the alternator. I was using defog + headlights + front/rear wipers + radio and it surged; turn one thing off and the surging stopped immediately, and you could sometimes turn the accessory straight back on and it would stay settled.

I'll get the battery checked, and the alternator's output, and I'll run some cleaner through the fuel system. Thanks!

 

[312 Sprinter]

If it is dependent on the load on the alternator I'd start off by checking/cleaning the the earth points. If it has a high resistance joint somewhere it'll corrupt the readings from the sensors. There is no point in getting into swapping components until you're sure the basics are correct.