My car has an intermitent idle problem. When it warms up the idle will surge from 600 to 1300 rpm, up and down. If I put it in gear the problem stops but when driving the car there seems to be a hesitation when accelerating. The problem may not appear for a week or more then it comes back. When the problem is gone the car drives normal. The engine wiring harness has been replaced. My dealer says he has never heard of this. Can you help me resolve this problem

Idle "surges" and the associated symptoms are actually quite common with these engines. But a variety of issues can cause them, making troubleshooting difficult.

I too, have the annoying idle "surge" and have solved just about every other problem in that car over several years...EXCEPT the surge!!!

The most prevalent cause is a vacuum leak. Check all vacuum hoses for integrity or any that are loose...they should all fix snugly enough to not be able to pull them off the connections without some muscle.

The MAS (or mass air sensor) is another culprit.

I've also read about the neutral/backup safety switch and the intake manifold leak as possible causes. However, I don't recommend "throwing parts" at this problem, as MB parts can get pricey, and it's frustrating to do so and still not fix the problem.

Do a search on "surge" or "surging" and you will find that many others here have struggled with this!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Have you checked for any codes stored? My surging problem was associated with a faulty throttle actuator and cruise amp. Rather expensive but its fixed nonetheless. Arthur Dalton has posts on how to build a code reader and how to retrieve the codes....a tremendous help on these cars. Diagnosis is key to saving $$$$.

I am told by the dealer that will not be any codes if the service light hasn't come on. Where would I find the info on the code reader?

There are other codes that can be stored without the check engine light coming on. Do an advanced search on this forum for "Arthur Dalton" and "tool build" you should find it. It takes about $10 of RadioShack parts and once you have it built you can read additional codes. Do you have the 16 pin diagnostic block on your car? It should be near the battery, perhaps with a small plastic cover over it.

I am not sure. I will look tomorrow morning and get back to you. Thanks for all your help.

Your dealer is either incompetent or lying.

Or both.

Surging idle on M104 engines is very common. All MB techs have surely seen dozens and dozens of cars with this problem.

Further, the Check Engine Light only illuminates when smog related faults are triggered.

The diagnostic module holds a ton of codes that won't cause the CE light to come on.

There can be three causes of the problem:

1. vacum leak
2. bad throttle actuator
3. bad mass air meter

Make Arthur Daltons code reader and follow his directions in using it. The codes may lead you to the solution.

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Paul S.

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".

What a great resource site. Thanks to all that have provided the information. I have found the diagram for the code extractor that Arthur Dalton posted. I will get this put together in the next couple days and let you know what codes I get.

I kinda felt the dealer didn't want to get involved with the problem. I am inclined to think the problem may be vacuum related. This morning I tried to operate the throttle from under the hood and I got no response even wide open. I went into the car and gave it gas and it worked. I did notice the ASR light was on. I restarted the car and it went off. As I understand the throttle is vacuum operated however when I looked up the throttle actuator on the Benzbin website it says the throttle actuator is only for cars without ASR?