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Old 03-26-2002, 10:35 PM
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Join Date: Feb 2002
Posts: 32
Unhappy rough idle

Dear all,

I now owned a 95 SL for more than 2 months and it is a great car. It has 60 K miles on it. When i got the car, the 60K miles maintenance was performed. New spark plugs, new oil, oil filter, air filter, fuel filter, transmission fluid and filter. New engine mount was replaced. The car idles rough especially in the morning when cold. The RPM fluctuate between 800-1000. If I press the gas pedal steady, sometimes the idle fluctuate itself between 1000-15000 rpm. Once engine is warm, i still feel significant vibration, more so in neutral than in drive with slight but definite fluctuation in RPM. The car drive very well without any hesitancy.
I took the car to 2 independent shops. There was no fault codes. They change spark plugs without any improvement. I put in 4 techtron and 2 redline fuel injector cleanser without any changes. The wiring harness was inspected and said to be OK.

What else should i be looking for?
thanks in advance
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Old 03-27-2002, 02:43 AM
David C Klasse's Avatar
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Was the fuel filter (filters?) replaced at the 60k service? The MB Maintenance manual says that it is recommended, but not required. I had mine changed at 80k miles and it was definitly time to change! Symptom was a rough idle also. I'd give that a try. Good luck!

You say that one engine mount was changed? Or do you mean all were? If yes, then I'd have the other one changed as well.
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Old 03-27-2002, 10:35 AM
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Join Date: Feb 2002
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thanks for the reply. Yes, the fuel filter was changed and i think both the engine mount were also changed. One thing that i know was not changed was the tranny mount. Anything else i should be looking for or i should head to the dealer?
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Old 03-27-2002, 11:09 AM
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Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
Distributor cap, rotor and wires.... how do they look?

Your fuel pressure regulator could also be the culprit. Does the car start up immediately when cold, or does it take a few cranks?


'91 300E, 208k miles
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Old 03-27-2002, 11:19 AM
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i was told that cap, rotor and wiring looked ok. i took about 2-3 second to start when cold but immediately when warm. how do we check fuel regulator?
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Old 03-27-2002, 11:25 AM
pmizell's Avatar
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Join Date: Jun 2001
Location: Houston, Texas
Posts: 838

Visit this thread:

300E rough rough rough idle

Lots of possible causes including the infamous wiring harness on your '95 model. Has it been replaced?

Good luck...... use the search function, there's a wealth of info on rough idle.


'91 300E, 208k miles
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Old 05-26-2002, 02:13 AM
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Join Date: Feb 2002
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still rough idle

wiring was ok according to independent shop. the engine still have rough idle. the mechanic told me that one of the 12 cylinders may be misfiring. engine check light never goes off. the vibration seems to come from the left side of the engine. he told me it is normal with 60 k miles car. it that true or it is time to go to the dealer? any other suggestions?
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Old 05-26-2002, 02:56 AM
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Join Date: Apr 2001
Location: Southern California
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Has your mechanic...

...performed a leak down test? Often the most obvious reason for rough idle (leaky valves) is overlooked. Even on a relatively new engine (and your 95 with only 60K miles is NEW) valves can leak for a number of reasons. So have your mechanic check this out (if he hasn't already done so).

And enjoy your car. It is a beautiful piece oof mechanical engineering. Good luck.
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Old 05-26-2002, 03:22 AM
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thanks for the reply. i see that you are in so cal. can you recommend me a shop in LA? BTW, is valve thing an expensive proposition? mike
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Old 05-26-2002, 11:15 AM
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Location: Florida / N.H.
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If the CE light never goes off, then there is a fault code stored ... and that is where you go to start diagnoses .that's what the diagnostic module is there for .
go get the code ....
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