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Old 07-06-2005, 06:26 PM
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Join Date: Jul 2005
Posts: 4
Poor Idle

I have a 85 500SL that will not idle below 2000 rpm. I replaced the o2 sensor and then it would not crank. Disconnected the o2 sensor and it goes back to very rough idle below 2000 rpm. Runs great above that up to 100 mph. There was heavy carbon on the 02 sensor. Acts like it is flooding out at the low rpms. Any suggestions???


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Old 07-07-2005, 09:06 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153

That car as well as most models after 1980 has an electronic idle speed computer. That computer gets power from the overvoltage relay just above the fuse box, I would start there!
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Old 07-07-2005, 02:43 PM
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Join Date: Jul 2005
Posts: 6
mr. MB DOC

I have a w124 300E 24valve yr1990. my car initially has an overload relay with two fuse inside and recently replace a new relay with only one single fuse.

the shop told me they are same. are they correct? Is this single fuse relay suit my engine? advice pls.
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Old 07-08-2005, 09:07 AM
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Join Date: Jul 2005
Posts: 4
85 500 SL Poor Idle

ThanksM.B. Doc, I will check that out and replace it. Thanks again.
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Old 07-09-2005, 10:53 PM
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Join Date: Jul 2005
Posts: 4
85 500 SL Idle Problem

I cannot find the fuse box or the idle sensor. Guess I do not know where to look. I took the car out today and ran it about 40 miles at up to 100 mph. It seems to idle better now that it has run some. But still not smooth. The encomy guage is in the red most of the time.The vacuum lines are all very hard, could it be a vacuum leak??

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Old 07-10-2005, 12:00 AM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Originally Posted by Ken Beard, Sr.
...The encomy gauge is in the red most of the time. The vacuum lines are all very hard, could it be a vacuum leak??
Most definety, yes, a vacuum leak will do bad things to an idle. An engine cannot idle at low rpm with a vauum leak. A vacuum leak lets extra air into the manifold, and messes up the air/fuel mixture ratio.

Buy or borrow a MityVac and track down your leaks leaks. HINT: On our diesels, the soft rubber connector that harden up are very common sources of vacuum leaks.

Best Regards,

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