85 500 sel Idle Problem, up amd down
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1985 Mercedes 500 SEL
85 500 sel Idle Problem, up amd down
I have a 85 MB 500 sel with an idle problem. Starts easy, but will not hold constant idle, instead it goes from approximately 500 to 900rpms, it continues idling this way, but sometimes will idle to low and die. When I give it pedal it runs fine, no missing.
Video: http://www.prestonimages.com/mbidle.htm
Video: http://www.prestonimages.com/mbidle.htm
Last edited by ChamaRiver; Mar 8, 2014 at 10:50 PM. Reason: removed reference to other video
Thread Starter
Junior Member
Joined: Jun 2012
Posts: 40
Likes: 1
1985 Mercedes 500 SEL
85 MB 500sel Idle Problem
Hi Rapaman, I cleaned the idle air control valve with carb cleaner, even though it was working fine before the cleaning. When power is put to it, it functions as required. I did some adjusting of the allen screw in the air mixture tower and have got it to where it does not idle up and down anymore.
But now, when I turn on the key to start the engine, it starts with no problem, idles at around 750rpms, revs smoothly, no missing, but the idle begins to climb to about 1200rpms as the engine gets warmer, in about 4-5 minutes. It then holds at 1200rpms and does not come back down to normal idle of 750rpms. I had connected a small bulb to the two wires connected to the idle air control valve and noted that no power was going to it, except one time and that time the valve was activated and the engine dropped to normal idle speed of 750rpms.
I have several Idle Control Units, which I have switched out, and same with the Fuel Relay. All are working ok, but the power going to the Idle Air Control Valve is most of the time not getting to the valve. Whenever the valve does get the power the idle is as normal.
But now, when I turn on the key to start the engine, it starts with no problem, idles at around 750rpms, revs smoothly, no missing, but the idle begins to climb to about 1200rpms as the engine gets warmer, in about 4-5 minutes. It then holds at 1200rpms and does not come back down to normal idle of 750rpms. I had connected a small bulb to the two wires connected to the idle air control valve and noted that no power was going to it, except one time and that time the valve was activated and the engine dropped to normal idle speed of 750rpms.
I have several Idle Control Units, which I have switched out, and same with the Fuel Relay. All are working ok, but the power going to the Idle Air Control Valve is most of the time not getting to the valve. Whenever the valve does get the power the idle is as normal.
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Posts: 40
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1985 Mercedes 500 SEL
Vac leak maybe
Hi Xlimodriver, I sprayed carb cleaner (with fire extinguisher ib hand) all around the vac hoses and got no rpm changes from engine, so believe I am good with that one. Last summer I replaced most plastic vac lines and rubber Xs/Ts/and Ls.
Also, I would think that if it was a vac leak then it would show at first start up with idle. But as stated before, starts easy, idles smooth, and only after a few minutes the idle begins to climb from 750rpms to 1200rpms. Beyond that all is normal.
Thanks for you input. Dan
Also, I would think that if it was a vac leak then it would show at first start up with idle. But as stated before, starts easy, idles smooth, and only after a few minutes the idle begins to climb from 750rpms to 1200rpms. Beyond that all is normal.
Thanks for you input. Dan
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Joined: Jun 2012
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1985 Mercedes 500 SEL
Idle Speed Control Test
Tried moving the plug on bottom of Idle Speed Controller, rpms did not change. With car running, disconnected the Idle Speed Controller, plugged in a 2nd one and nothing. I then (engine running) plugged in a 3rd Idle Speed Controller and the small bulb connected to the Idle Air Valve wires lit up and the engine died.
I then tried the same procedure, with all three Idle Speed Controller units and got the same results. So the first two do not turn on the Idle Air Valve, but I can start the car without putting my foot on the pedal, but the car will idle at 12rpms.
And when I use the 3rd Idle Speed Controller, the little bulb comes on, which means power is going to the Idle Air Valve, but the car will not start without putting the pedal down a little, and if I take my foot off of the pedal the engine dies.
I then tried the same procedure, with all three Idle Speed Controller units and got the same results. So the first two do not turn on the Idle Air Valve, but I can start the car without putting my foot on the pedal, but the car will idle at 12rpms.
And when I use the 3rd Idle Speed Controller, the little bulb comes on, which means power is going to the Idle Air Valve, but the car will not start without putting the pedal down a little, and if I take my foot off of the pedal the engine dies.
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Thread Starter
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Joined: Jun 2012
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1985 Mercedes 500 SEL
This is an update. As the car ran fine, and the idle issue continued with the Idle Speed Control Unit still on the car, I decided to go ahead and get a new one and change them out today (1/15/20). Nothing changed, the idle still runs high. I have checked the Speed control valve and it still works when 12 power applied. So no I am at a total loss as to the problem. Any ideas?
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Joined: Feb 2023
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1989 560SL
Near identical problem with my '89 560SL. Intermittent high idle at 1200. Voltage reading at the idle control valve is all over the place when the idle is high (rapidly moving from 0.5 to 4.5v and everything in between) Then on another day, it idles at 650ish, and the idle control valve shows a solid 4v continuous. I just ordered a new Idle Control Relay and hope I have better luck than you. Did you ever find a solution?
