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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53944
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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1985 M 380SL....runs gh idle at most times, will drop down

Customer Question

1985 M b 380SL....runs high idle at most times, will drop down to normal at times and now it is idling from 300 -600 rpm when stopped or in park.....not sure if its idle relay, idle control valve or???? been doing this for many months. idle is all over the place at times
Submitted: 1 year ago.
Category: Mercedes
Expert:  Marty replied 1 year ago.

Hi, and welcome back. It's probably best if I call and talk with you about this on the phone. If you will accept the additional service offer for the phone support I can call you tomorrow and help you with this.


Customer: replied 1 year ago.
i would prefer it by mail...very hard to talk at work
Customer: replied 1 year ago.
how can i view your response?
Expert:  Marty replied 1 year ago.

Okay, the first thing I need to know is whether the rpm increases to 1500 rpm when you disconnect the idle valve. I believe you mentioned that it did not increase.

Customer: replied 1 year ago.
Hi Marty....i started the car and it stsrted to " pulse" from300-600rpm. I unclipped the wire to idle valve and it went to 1100rpm.put clip back on and ran perfect. When car is racing at high rpm and i unclip idle valve nothing chages ,it will continue to stay high.sometimes it will drop down but most times it doesn't change. Car can run fine and then after a few miles it starts again. The problem with it "pulsing" is something that started a few months ago....other problem has been for years....ready to give up...runs great at times
Expert:  Marty replied 1 year ago.

Okay, do you know how to adjust the fuel mixture?

Customer: replied 1 year ago.
No but can try if you can help....i have noticed recently the car smell "rich" . i did have to replace the fuel relay last year but not sure if that has anything to do with it.....problem i have for next few days is i am working so not easy to work on car untill maybe Tuesday. But i can try to do what i can . i am lacking in electronic tools to check elect problems and I'm no mechanic....ty
Customer: replied 1 year ago.
Started car this morning and it started "pulsing" 600-900rpm. I removed clip from idle valve and it raced to 1200rpms, put clip back on and car dropped down to normal ,no pulsing and ran perfect for ride to sat for about an hour and restarted it....went to 1300rpm immediately and i unclipped idle valve and nothing changed. Waited 30seconds and reclipped idle valve and dropped down and started pulsing again . i unclipped again and idle back up.....reclipped and back down to pulsing with rich smell of fumes
Customer: replied 1 year ago.
I think i found the adj screw for fuel mix.....i started car and it ran high idle....turned screw on top of air cleaner counter clock and lowered till rpm lowered ....not sure what rpm should be in park.....i haven't driven home from work yet to see how it runs.....i assume my idle control valve is working? If car is idling fine and i remove clip car will race high....but if car is idling high and i remove clip nothing changed....or do you think the adjustment to fuel was the problem all the time? I had idle relay replaced when i first got car in 09.....but trouble returned soon after......30 percent of time car runs fine.....other times not so..
Expert:  Marty replied 1 year ago.

You need to adjust the mixture for highest engine rpm. Click on the attachments below.

Expert:  Marty replied 1 year ago.

When the car is idling high I would suspect a fault with the idle control unit or the overvoltage protection relay or the connections to the idle valve. First I would tighten the connections at the valve. Next you would need to remove the cover on the back of the connector on the idle control unit so you can probe the terminals for power when the car is idling high. You should have 12V on the red/blue wire at terminal 2. Power comes from the overvoltage protection relay. If you have no voltage you will need to replace the relay. If you do have 12V when the idle is high I would replace the idle control unit. I've also found that poor connections at the control unit can cause erratic idle speed.