Inherited 1985 380SL with fast idle problem, please help! - Mercedes-Benz Forum
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#1 (permalink) Old 01-15-2008, 03:55 PM
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Post Inherited 1985 380SL with fast idle problem, please help!

I just inherited a nice one owner 1985 380SL with 138k miles. Car appears to be in excellent mechanical health but I have only been in it once. It hasn't been driven much in past 6 years due to former owner's health. I can't say for sure what has been serviced and what has not except for the following:

Car starts right up but idles very high at about 1900 rpm and while warming up idle fluctuates up and down but engine will not kick down to around 900 which is where I am guessing it should be warm. I have been reading about people who have similar idle issues and what to do but I am not that familiar with these cars having driven diesels most my life. If anyone can provide me with some basic step by step problem isolation tips to start with I would be grateful. I would like the pleasure of diagnosing and fixing the car on my own if I can help it. If I need to start with throttle linkage, temp sensor for the idle speed unit, or the idle speed control amp I can handle it if someone could tell me where to look!

Thanks so much. These forums are very helpful and full of knowledgable helpful people.

I can be emailed directly at realtor4u@hotmail.com
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#2 (permalink) Old 01-15-2008, 04:43 PM
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Car starts right up but idles very high at about 1900 rpm and while warming up idle fluctuates up and down but engine will not kick down to around 900 which is where I am guessing it should be warm.
I'm going to go with either one hellacious vacuum leak, or the auxiliary air valve is stuck open. Either way, you're getting too much air. If the vacuum hoses appear to be solid and all connected where they should be, the AAV should be your next stop.

Edited to add: we typically don't e-mail responses to people. That kind of defeats the purpose of a forum. What happens to the next poor guy with high idle on his 380SL? Much better if he can do a search and find it here.


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#3 (permalink) Old 01-15-2008, 05:07 PM
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Where is the air valve located to check and how do I do so? Are there obvious vacuum leak areas to look for? I guess I don't know what I am looking for. thanks for the assistance
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#4 (permalink) Old 01-15-2008, 05:37 PM
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Here is what they look like and how I tested mine.
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#5 (permalink) Old 01-16-2008, 09:52 AM
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Smile

Search is your friend...

380SLC Idle Speed Discussion

In the middle of the post you'll see a link to the section of the service manual that deals with idle speed control.

Good luck.
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#6 (permalink) Old 01-16-2008, 06:24 PM
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Thanks for the info...I will check these items and start ruling things out one by one.
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#7 (permalink) Old 01-20-2008, 02:23 PM
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Ck for a blown fuse on the over voltage relay

ASE Master Tech shop owner,93 400E,85 380sl,69 Nova SS
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#8 (permalink) Old 01-20-2008, 03:54 PM
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Ck for a blown fuse on the over voltage relay
I will check all the fuses once I get the car home...is the overload protection relay a black box located within the fuse panel or somewhere else?
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#9 (permalink) Old 05-16-2008, 07:43 AM
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380SL HIgh Idle solution!

After a lot of hassles and research my problem is finally resolved. If you have a high idle issue, start with the idle speed control valve located in front of the air cleaner-it is a slender silver tube with an electrical plug at the front-you can't miss it hiding just under the front edge of the air cleaner assembly-first check that all the hoses that plug into the air cleaner and the valve in various places are not loose, missing or cracked causing an air leak-you would not believe how a small cracked hose can cause irratic idle in these cars. I bought a used idle speed control valve on ebay for $20 just to see if this indeed was my problem after replacing the idle speed control box behind the glove box to no avail. Once the valve was replaced and the hoses properly sealed, the car runs perfectly and starts quickly-what a difference this little part makes. They are not cheap but can be purchased new through Performance Products which I recommend. MBCA members get a 15% off discount and for $200 you'll solve your idle woes. I went ahead and tuned my car up while I was at it with new plugs gapped to .35 and new plug wires, cap and rotor. Total cost was under $250 for everything and about 3/4 day of labor at home in the garage. The hardest project so far was restoring the proper operation of the air flaps for the defrost and center vents in my car-bleeding knuckles are common-but now I have working climate control which I like! Oh, and replacing the soft top was really fun too-
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#10 (permalink) Old 05-16-2008, 07:55 AM
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^^
Vacuum, who would have thunk it.

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