Will not idle.

[plnnsmpl]

Hello, long time stalker of the site.

Was able to finally pickup a 1988 560sl - was driving great than one day it died. Every since it will not idle at all, can mange to get it running with some assisted gas pedal pumping once it runs if you hold the throttle it runs good.

As soon as u let off the throttle the car dies. I have read alot about the fuel system on this car.

Things to note:

- a/c was repair less than 2 weeks ago
- idle air control motor resistance is with spec at 4.3 ohms,
- have 12 volts at the idle air control unit

I am a technician by trade, but new to mercedes. Without OBD2 i seem to be lost not to mention the crazy fuel system this car has on it.

Hope this helps and we can fix my car.

Thanks in advance

[meltedpanda]

If you are certain the idle control valve is working properly then as a start you need to check the system pressure and warm control pressure Also check for sticking or binding airflow plate in intake. Does it die with the AC on and off?

[Frank Reiner]

pinnsmpl:

The fault may be in the idle control unit.
Tests: 1) unplug connector from idle air valve (IAV); start engine. Idle speed 1200 RPM or more? 2) If yes, reconnect IAV while engine running. Engine stalls? If yes, the IAV is operating, and the controller is sending a fixed current to the valve, rather than a varying current. With no current to the valve, the valve is full open; with increasing current, the valve closes.

Have you discovered the overvoltage protection relay (OVP)? There is a fuse on the top of that relay; when that fuse has blown the idle symptom is usually the opposite of what you are experiencing; i.e., a high idle.

[plnnsmpl]

Originally Posted by Frank Reiner

pinnsmpl:

The fault may be in the idle control unit.
Tests: 1) unplug connector from idle air valve (IAV); start engine. Idle speed 1200 RPM or more? 2) If yes, reconnect IAV while engine running. Engine stalls? If yes, the IAV is operating, and the controller is sending a fixed current to the valve, rather than a varying current. With no current to the valve, the valve is full open; with increasing current, the valve closes.

Have you discovered the overvoltage protection relay (OVP)? There is a fuse on the top of that relay; when that fuse has blown the idle symptom is usually the opposite of what you are experiencing; i.e., a high idle.

With the idle air valve disconnected, the car will not stay running at all. The car will not idle at any speed with out assist from the throttle.

The throttle plate moves up and down freely, however with the car off and pushing the pedal the plate doesn't move is this normally? Vacuum controlled?

I havent checked the overprotection relay yet. where would i find that?

When the car is running it seems to make no difference if the a/c is on or off. the car still will not idle.

I don't think that fuel pumps are bad as it runs good when keeping it going with the throttle. That also eliminating spark as a issue.

So i was reading about disconnecting the fuel line at the a/c line? I can't figure out what is going on with the fuel lines at the a/c.

Thanks for the feed back. Can't wait to drive this car.

Side note: Cars was driving great while going down the road than bam shut off and now will not idle

[Frank Reiner]

Originally Posted by plnnsmpl

With the idle air valve disconnected, the car will not stay running at all. The car will not idle at any speed with out assist from the throttle.

The throttle plate moves up and down freely, however with the car off and pushing the pedal the plate doesn't move is this normally? Vacuum controlled?

I havent checked the overprotection relay yet. where would i find that?

When the car is running it seems to make no difference if the a/c is on or off. the car still will not idle.

I don't think that fuel pumps are bad as it runs good when keeping it going with the throttle. That also eliminating spark as a issue.

So i was reading about disconnecting the fuel line at the a/c line? I can't figure out what is going on with the fuel lines at the a/c.

Thanks for the feed back. Can't wait to drive this car.

Side note: Cars was driving great while going down the road than bam shut off and now will not idle

On the first highlight: I would believe that the IAV is stuck closed.

Second highlight: That plate is not the throttle plate; it is the airflow sensor plate. It moves in response to increasing air flow into the engine, and in turn moves the fuel control plunger (thru a lever) in the fuel distributor (FD). It is the mechanical equivalent of an electric mass airflow sensor.
The throttle plate is buried at the front of the engine, behind the distributor. Follow the throttle linkage.

Third highlight: The return fuel line from the FD passes through a fuel cooler that forms a jacket around the A/C low pressure line.

Did the engine first shut off while you were driving with the revs up, say in the area of 2000 RPM, or did it die when coming to a stop with the throttle closed?