300SE Idling problems

[Bern]

300SE 1990. Beautiful car with 114,000 miles but has just started to run rough . I see this problem has been mentioned in the forum by other members, but I don't know where to start. Symptoms are: Difficult to start and coughs quite a bit. After 3 or 4 attempts, it will tick over at 450 -500 rpm firing on 4 or 5 cylinders. I nurse into life. It sometimes peters out at traffic lights or when I stop suddenly. Sometimes the speed suddenly picks up to 850-900 rpm and seems ok. I put it into Merc main dealer and they replaced the distributor cap and rotar arm and adjusted the mixture saying it was running rich. It now seems to be too weak. I also had a red coloured relay replaced up near the battery just in front of the screen. Prior to this the ABS light was on and I was told the faults were related. I also had a vacuum pipe replaced somewhere under the manifold. None of this fixed the problem. The car also runs on LPG and for 25,000 miles I've had no problems, but now it cuts out at low speed on gas, so I run it on petrol to be safe. I'm told there is an air valve someewhere that may be sticking and I should squirt it with WD 40. Any help appreciated

 

[MondeoMan]

Before finishing reading this I suspected the idle control valve, but I'm surprised the dealer didn't pick this up. Cleaning it out with carb cleaner (not WD40) might help - it's usually cylinder shaped with a single electrical connection, with 2 small pipes leading into both sides of the throttle butterfly.

 

[Bern]

Thanks Mondeoman.
I'll tackle the Merc Main dealers along the lines you suggest

 

[Sanjay16]

The culprit could be the fuel pump relay - this a not just a simple relay, it is a box with complex circuitry which controls other gizmo's in the car aswell.
It may look like a small black box but, don't be deceived.....try replacing this and your cut out problems may dissapear.

 

[television]

There are 3 parts to the system,
The throtle position switch tells the main ECU that the engine is at idle, and tells the idle control relay/ ECU to work, the idle control relay / ECU tells the idle control valve the correct RPM for idle.

The main ECU controls all above idle, if the idle system is not working, the fail safe is a very high idle most of the time. A dry joint in the idle control ECU can cause these problems. The swich for idle is either in the butterfly aperture(micro) or on the throtle cable.

Malcolm

The Idle ecu cause the most problems