Uncontrollable Idle - 420SEL

[Bigraven]

Hello everyone, I've recently encountered a problem with my '86 420SEL. When I get it started, it goes into a frenzy of an idle. It tops at 2500rpm. I've replaced the Idle control valve and switched out the fuel pump relay. Yet, the problem still continues. I'm in the process of taking out the idle control unit, but first I have to find one in my local salvage yard. A few question that have risen in my search for the mysterious idle problem. 1.) Which cars have the same Idle control unit as my '86 420SEL and where is the unit located on those other cars? I know that the unit is located in the passenger side footwell. 2.) What other problems, besides the Idle control unit, may be causing this very high idle? If any help, the car has 206,000+ miles on it. Any help would be much appreciated. Regards, Allen

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[Untertürkheim]

When is the last time you changed your fuel filter?

[BENZITCH]

Check your OVP relay. It has a red 10 amp fuse on it. Sometimes they blow, but usually the over voltage protection relay goes bad.
Bruce

[Bigraven]

Originally Posted by Untertürkheim

When is the last time you changed your fuel filter?

The original owner said he replaced the fuel filter about a year before I purchased it as a regular maintenance that he did for the car. From what I can tell, the fuel filter, pump, and relay have all been replaced and are in excellent working order. Car was purchased for $300 on August 15, 2007. Everything is PERFECT except for this high idle.

@BENZITCH
From various other forums that I've asked this same question, nothing leads to the OVP relay. From searching this forum and many others, some sort of dash light will be lit when the car is running (correct me if i'm wrong). There is no light on the dash, so i'm doubting that it could be the ovp relay.

Edit: I checked the fuse in the OVP relay. It wasn't blown out or shown any signs of wear and tear over the years. I've also replaced the fuse in it as well. Still no luck.

[BENZITCH]

Try removing the OVP and see if the idle changes. If the idle remains the same than I would think it is still the OVP.
Have a friend with a similar car? Swap his out and see.
As far as the lights do you have an air bag and abs?

[Bigraven]

Originally Posted by BENZITCH

Try removing the OVP and see if the idle changes. If the idle remains the same than I would think it is still the OVP.
Have a friend with a similar car? Swap his out and see.
As far as the lights do you have an air bag and abs?

I'll see if I can get an OVP from a buddy of mine and test it out, who knows, it just might help it. I do have an air bag and abs.

[tbjcc001]

Have you resolved this issue? I'm having the same problem with my 86' 420 SEL & would appreciate the benefit of your response......

TerryJ

[Bigraven]

Originally Posted by tbjcc001

Have you resolved this issue? I'm having the same problem with my 86' 420 SEL & would appreciate the benefit of your response......

TerryJ

Hi Terry,

I've accumulated a collection of 4 idle speed controls, 3 idle control valves and 3 ovp relays; none of which seem to be my problem. I've also replaced the following: ignition control module, cap and rotor, spark plug wires, spark plugs, fuel pump relay, fuel pump, vacuum hoses, and a new ABS unit (ABS brakes weren't functioning as they were supposed to). I'm getting so many parts from the junkyard I could build an entirely new engine. Although, I have yet to replace the throttle position switch. That is my next step.

None of the replacement parts seem to have effected the idle of my benz. At start it goes for a straight shoot to 4000rpm and keeps going. If your idle isn't as erratic as my idle is, then possible problems with your idle are in descending order with most common problem first:

1. Idle control valve (piston inside gets stuck, try cleaning in a tub full of carb cleaner and leave overnight)
2. Idle speed control (little box inside the passenger footwell)
3. OVP relay (little silver box with a red fuse inside with a "10" on it)
4. Throttle position switch
5. Worn injectors
6. Broken/brittle/missing/disconnected vaccum hose(s)

Sorry for my lack of information on this. I've been at this for quite some time now with minimal success. I have, however, succeeded in replacing a majority of interior blemishes through my scavenging across junkyards and finding parts for my '87 diesel wagon.

[novone]

Has any one removed the cruise control and throttle linkage rods? It may be that they are reversed at the control unit for the cc. I've experienced a similar start and straight to very high rpms problem.

[digital10]

I have the same problem with my 1986 420 sel. did u fix it? Can u tell me how?

[victan]

Gentlemen,

Test your COLD START VALVE! the described condition for RPM variation is normally caused by an open position stuck cold start valve which enables a steady stream of fuel to be injected into the intake manifold. Suitable remedies are: i) discontinue electrical source; ii) discontinue fule delivery and see if variation still occur.

Best,

Vic