Hi. I'm new here. Have been given a 1991 W126 300SE by a friend and am slowly breathing life into a car that has stood for many years. Some suspension and brake work and she now drives but is very difficult to start, often requiring 10 minutes or more of the ignition being on before an attempt can be made. Once started she runs fine for 3-4 days and will then suddenly stall. Start up afterwards is immediate but she won't idle without pressure on the accelerator pedal. I have replaced the fuel pump and filter, the over volt protection relay and the idle control valve and re-soldered the circuit board in the fuel pump relay. Yesterday it did it as I got home so I removed the air filter and started her again. What I observed was the ICV clicking and it time to this the plenum in the throddle body was moving up and down, almost like a heart valve beating!. Revving the engine to keep her going would produce a hesitation, also in time with the beating heart! So I disconnected the electrical feed to the ICV and the beating heart stopped, the engine revved freely (no hesitation) and idled, albeit at around 400rpm and roughly! It is obviously electrical and I am guessing to do with the signal to the ICV but has anybody experienced anything similar or know where to look? If I leave the car overnight all will be well in the morning for a few more days and then it will happen again

The Idle Speed Control Unit (ICU) is located in the right footwell. Remove the right front carpet, and the angled footrest panel that is under the carpet.
There are five control boxes in the space under the footrest panel. The ICU is at top center. At idle the ICU supplies a current of less than 1000 mA to the idle valve, and that current is smooth, rather than pulsed. The pulsing which you describe is erroneous. Even if the ICU is itself OK, there are, unfortunately, quite a few input signals to the ICU which could be in error.
If you can obtain a used ICU, installing it is the simplest first step.

Hi Frank. Thanks for the very useful info. I will check the ICU tomorrow to see if there is an obvious problem. I may have difficulty getting another unit without purchasing it new as the car is in Malaysia and there are only a few of these left around the place here and parts are hard to come by used. I assume the ICU will be fed from various sources including the Lambda sensor and crank position sensor and MAF (if it has one??). I am wondering if I can put a meter across the pins to see if the pulse originates before the ICU or only after it? ED

If you will PM me with an email address, I'll scan a manual page that shows all the connections to the ICU. If I attempt to post a manual page on this site via an attachment the resolution is too low to be usable.

Hi Frank. Thanks for the offer of the info page. How do I PM from here? I took a look yesterday under the kick plate in the front passenger footwell but only found the AC controller and the control box for the alarm, no ICU! I had to access the AC blower above as it is squeeling and I took a look there and also found nothing. I have looked between firewalls on bulkhead and apart from fuse box, Fuel Relay OVP etc, nothing else there. Took up the carpet in driver's side footwell but nothing there except a couple of lose wires that plugged back into a switch at the base of the accelerator pedal. Only place left now is under the dash on driver's side. Will take a look there tomorrow or Thurs. Thanks for all your help. BTW, any way to isolate the unit and send a fixed signal to the idle control value?

Also may want to check for a faulty coil. Not sure if that is your issue, but we were told to check this and that and we checked everything, but nothing helped until someone told us to replace the coil up by driver front of the engine.

Sounds strange, but when we were having problems with my moms '88 300SEL, turned out it was indeed the coil that was going bad. It would be difficult to start at times and sometimes when it would start as soon as you pushed the pedal it would stall. New coil and its been fine ever since and that was about 2 years ago.

EdJB, Re: PM
When you are logged on, click on my name in a post, and a menu will open.

On the matter of a fixed signal, I suppose a circuit with an appropriate potentiometer could be devised. The ICU actually sends a fixed voltage, PWM signal. Using a simple meter, we read a mean amperage; with an oscilloscope we would see the pulses.

Hi 86560SEL. I will check the coil but as the car runs normally most of the time and a coil fault would probably cause a continuous problem it may not be at fault. Always woth a look though! Thanks for the info

No problem! Hope you can figure it out, I know how frustrating it can be.

Good luck!

Hi Frank

I have been unable to locate the ICU and am now told that it may be up behind the dash binnacle in a pretty hard to get at place. Having said that, the problem hasn't recurred of late so will wait a while and see what happens. The bad starting is down to the fuel distributor which doesn't seem to bleed properly! If I crack a fuel union the engine will instantly start but as soon as I close the union again it will stall. I have taken to turning on the ignition and leaving it for 10 minutes before attempting to start it, thereby letting fuel pressure build up. That works. I have ordered a replacement fuel distributor and hopefully all will be well now. Thanks for the help!

Hi
leaving the ignition on for 10 minutes is not good . the fuel pump gets activated for 5 sec only and then again when the starter is activated .

check the rotor for high resistance and the distcap for a broken carbon contact easily gets broken during a cap installation..

how did you diagnoise the faulty fuel disttributer ? I assume you checked the fuel return pipe is not kinked and that the fuel presssure regulator is not faulty?