[hoisinga]

I have a 1985 Euro Model 500sl that has been having some issues. I replaced the fuel distributor about two months ago and an idle air control valve in the last week. When the car is cold it starts up and drives fine for about 5 or less minutes. After a few stops, and the car returns to idle, the idle drops below 500. Often times it even drops as low as 100-200 rpms. My vacuum gage is usually pined to the left at idle and doesn't seem to move. I tried replacing the idle control unit as it appeared that it wasn't operating the idle control valve. Put a new ICU in and nothing. Returned it and got a idle control valve. Still nothing. Reading with a volt meter how is the idle control valve suppose to act. Should I feel/hear the valve turn on and operate with the key turned all the way over but not running? What else can I check on the car to try and determine this problem? The car seems to be running rich as well. may or may not be related to this problem as it has been running slightly rich for a few months. Any help is MUCH appreciated as I have been trying to get this car out of my garage and into storage but very inconvenient to drive as I have to keep my foot on the gas at all times in order to keep it above 500 rpms. If someone is able to help me get my car running well again I would be more than happy to compensate them for their time.
Thanks so much in advance guys
Andrew

 

[hchaugli]

Seems you have been trying the shotgun approach without luck. Have the same car, here are my comments:

"When the car is cold it starts up and drives fine for about 5 or less minutes"
Seems the warm-up regulator is doing its job.

" After a few stops, and the car returns to idle, the idle drops below 500. Often times it even drops as low as 100-200 rpms."
This is way too low, it is electronically controlled and should not fall below 600 rpm. The idlespeed controller or control valve failures usually results in high idle rpm.

"My vacuum gage is usually pined to the left at idle and doesn't seem to move." Suspect you have a vacuum leak.

"I tried replacing the idle control unit as it appeared that it wasn't operating the idle control valve. Put a new ICU in and nothing. Returned it and got a idle control valve. Still nothing."
These were not the problem.

"Reading with a volt meter how is the idle control valve suppose to act. Should I feel/hear the valve turn on and operate with the key turned all the way over but not running?"
The idle control valve operates at around 200Hz, you will not hear it operate. It uses 12V switched with a duty cycle less than 50%, average voltage when operating properly is 4-6V. I haven't checked, but suspect the ISC will not be active when the RPM pulses indicates a very low RPM.

"What else can I check on the car to try and determine this problem?"

1) I would try to chase down the vacuum leak, has something come off?

2) The exhaust gas recirculation could also be a possible problem, it opens on when the engine is warm.

Attached is a diagram of the engine controls.

 

[hoisinga]

If it were vacuum wouldn't it run poorly ll the time? It runs very well at idle and while driving when cold but as soon as i drive it even for a minute or two and come to stop a few times it always starts to act up and want to die. What related part should I look at related to exhaust gas re-circulation? I would assume you are referring to #37 on the diagram you posted. What does temperature valve 50c do and how do I trouble shoot to see if its working properly or not?
thanks again

 

[hchaugli]

One theory would be that you have a small airleak which makes the car run lean when warm, the warm-up regulator would enrich to near normal when active the first few minutes.

The Exhaust Gas Recirculation Valve (#37) is vacuum controlled and opens at an engine temperature of 50 degrees Celcius to mix some exhaust with intake air to reduce nitrogen emissions. It is located at the LHS front in the engine bay, see picture.
A leak in the steelpipe would provide a lot of extra air.

Could of course be many other things, maybe the mixture setting with warm engine is wrong (quite sensitive to a fraction of a turn) or the idlespeed control valve is stuck closed. This could be checked by disconnecting the plug, should get an idle of about 1800 rpm.

 

[hoisinga]

Even with the new valve in and unplugged the car still wants to stall. Is that a sign that it is not getting a signal from the ICU? What other reasons would affect the ICV to not operate?
thanks very much for your help

 

[rowdie]

From the diagram it appears #20 temp. switch signals the ICU. Could be a bad temp. switch/sensor?

 

[hchaugli]

I have measured the signals from the ICU as shown in the pictures, is has an average voltage of about 5,2 Volts. If the ICU fails or gets disconnected from the ICV you would get either 12 or 0V across the ICV. In both failure modes my understanding is that it will provide additional that results in an RPM of about 1800 to limp home.

It is easy to check the ICV, disconnect both terminals and apply 12V briefly, you should hear a solid click. Even better remove it and blow through, as you connect and disconnect, you should notice that it opens and closes. Unless you bought brand new ICU/ICVs, you would not be the first one buying defective used parts.

Have also attached an electronic idle speed wiring diagram, no harm in replacing the relevant fuses (#12 and #14), blown/corroded fuses can be hard to see. My 500 does not have the engine load switch (#22).

For the car to idle nicely you need the right mixture setting, the right amount of air and sparks at the right time. Since it runs nicely when cold and at medium RPMs, I would suspect mixture or amount of air. The engine elements that are time/temperature dependent are the coldstart valve (less than 30s), the warm-up regulator which enriches the mixture until the engine reaches about 80 Celsius (several minutes), the EGR valve (50 Celsius) and the idle speed setting (42 Celcius). The idle speed setting reduces the idle from about 900 to 650 when warm, and a failure here would result in slightly high idle when warm.

At idle the air measuring disk should rest nicely at it stop, and all air should come via the ICV and the EGR valve.

Does the engine abruptly switch from smooth to rough at low RPMs, or is it smooth? The latter could point to a too rich setting when warm. The mixture setting Allan screw is quite sensitive, rotating it 45 degrees CCW reduced the CO from about 5% to 0,6%.

 

[rowdie]

Attached is a diagram of the engine controls.

Thanks for the diagram. I need it for the thermo vac switches 37 & 37a on my 500SL.

 

[hchaugli]

You are more than welcome Rowdie, found it in some old MB documentation I got in 1985 and translated it from German to English as part of my car documentation exercise. Believe I now have a complete set of wiring diagrams for this car.

 

[hoisinga]

Is there a technique to determine if the temp sensor 42C is bad?

 

[hoisinga]

It abruptly switches from running well to wanting to die. The first few minutes the car seems to run great but as soon as you make a stop or two it will stop running well and start to shake and want to die out. Is this a sign of anything specific?
Thanks

 

[jsebastian]

Could the new FD have anything to do with this? My FD on my SEC, after I had removed it to clean, started up fine, but after driving it for a few minutes, barely got it home after stopping at a light. Figuring I had ruined it when I cleaned it, I replaced it with a spare and back to running well.

Good Luck,

John

 

[hchaugli]

Is there a technique to determine if the temp sensor 42C is bad?

When the engine is cold the temperature switch is closed, open when hot.

 

[rockcrusher]

Isn't that the oil temp switch?????? #20 I believe it is.

 

[hoisinga]

Can you physically see if the switch is opened or closed when the car is off? The car does not stay idling long enough to get out and look under the hood before it dies. Is there something specific I should look for that shows that the switch is open or closed? Is there a mode I can put the car in to get it into my shop tomorrow morning? I have to drive it about 10 miles to work and it is very difficult to drive without using two feet.
thanks

 

[hchaugli]

My understanding of the German source documentation is that this engine temperature switch actually measures coolant temperature. (Kuhlmittel)

 

[ctaylor738]

I believe that the temperature switch (20) is at the rear of the intake manifold. Disconnect it. If it has two poles, jumper them and see if that makes a difference. That would make the idle relay think it needed to supply additional air for a cold engine.

You can also try unplugging the vacuum hose from the EGR and plugging it.

 

[hchaugli]

Can you physically see if the switch is opened or closed when the car is off? The car does not stay idling long enough to get out and look under the hood before it dies. Is there something specific I should look for that shows that the switch is open or closed? Is there a mode I can put the car in to get it into my shop tomorrow morning? I have to drive it about 10 miles to work and it is very difficult to drive without using two feet.
thanks

You can't see inside the switch, but you can measure it with a multimeter or follow ctaylor738s suggestion. If the idle speed control system is working, bridging it would increase the idle rpm when hot.

Can you mechanically prevent the accelerator from reaching the rest position, e.g. with a rope or a tiewrap?

 

[hoisinga]

I unplugged the EGR vacuum line and plugged and taped it so it would not leak but there was no change in idle. I started the car and it started and ran great but as I sat for a period of about 2 to 3 minutes the car suddenly began to shake and want to die. When I got out the garage smelled very strong of possibly fuel. Any other suggestions would be much appreciated. Thanks so much guys

 

[hoisinga]

I was also looking for the temperature switch yesterday and did not end up finding it. Does anyone have a good photograph or description of what to look for? When "ctaylor738" said "If it has two poles, jumper them and see if that makes a difference. That would make the idle relay think it needed to supply additional air for a cold engine." Do I connect the poles on the plug/harness side or do I connect the two prongs on the temp sensor after its unplugged?
Thanks again