89 300E, W124, M103 High idle... SOLVED

[TnBob]

Ahh yes.. the infamous nasty high idle, in my case 1500rpm !

What amazed me was the number of threads here with no resolution.
Soo have to love the responses ....''i dont know anything bout your problem but ....''

MB was running horribly due to a split in the rubber boot under the fuel distributor.

While the old one was off replaced both the hoses that go to the Idle Air valve. Even with the FD off, the hose going to the block is no fun. It was brick hard so replacing was mandatory. Finally popped it with an 18" pry bar. Great time to renew the two vacuum connections that are under the FD too.

Started right up after replacement and then the fun began. Enlisted the aid of Frank with his extensive knowledge to help me solve and fix the high idle.

List of checks.
OVP relay. checked good.
Fuel pump relay. checked good.
Idle air valve. checked good but swapped. No change. Idle would drop to about 1k when the connector was removed.
Throttle position switch checked, rechecked. Both checks were good.
EHA current checked good. EHA swapped. If you dont have a vise to help get the adjustment screw cover off, the top of the radiator frame works great ! Adjusted 1/4 CW. No improvement.
All vacuum points checks with throttle body cleaner.
Fuel accumulator swapped, no change.
O2 sensor checked for ohm value and voltage. Both good.

All this to not be able to find any thing that improved the idle speed.

Coolant Temperature Sensor at the rear right of the M103 proved to be the culprit. Its a temperatue sensitive resistor. With a cold engine several sites indicated it should read 38 ohms to ground and across both leads., mine was reading just under 4000 ohms. Finally a BAD reading.

Added a 1K resistor to ground from the green/red wire which brought the max idle down to 1k but the idle was never stable... 1k to ~500 back and forth.

The proof was by adding a 39 ohm resistor to the green/red wire to ground. Cold idle was now ~850 and stable.

Final proof will be popping a new one is which I understand is easier with the valve cover off.

My Coolant Temperature Sensor is the two pin variety. I have no idea which lead would be the right one for the 4 pin.

While I was at this EVERY electrical connection that was opened was cleaned and dielectric grease added on reconnection.