I have been battling an idle control problem and finally replaced the Idle Speed Control and Air Slide Valve. After driving an hour the idle speed is much more stable but there is still an intermittent stalling problem. It always seems to have 2 symptoms when this happens.

It only dies at idle when hot and before it dies, the tachometer will show 2000 rpm at idle (like 2 times the actual engine speed) and the fuel pump relay will be clicking, sometimes so fast it’s like a buzzing sound. I can hear it, feel it and it gets warm. It usually starts up fairly easily but sometimes needs to sit but only for a minute or two. The fuel pump relay is 3 months new and I have replaced the fuel filter, plugs, distributor cap and rotor, timing chain, rails and tensioner within the last 2000 miles. Could it be related to the thermo time switch? How about the tachometer or related items? Any ideas are appreciated. My curiosity is turning to aggravation.

1985 SL500 Euro (107 with 117 motor at 99,000 miles. Has Lambda but no air pump.)

I do not know that much about it, my guess is though to replace both the fuel pump and fuel pump relay and double check the OVP relay fuses.

__________________
1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.

I replaced the OVP Relay a few weeks ago and it didn’t help. I replaced the fuel filter last year. I don’t think there is a fuel pump problem but will have it tested for amp draw soon. I think if it were the fuel pump it would die or bog at other than idle. I do wonder if there is a lot of junk in the tank that the fuel filter could quickly clog. Still seeking a fix…

Steve
’85 500SL Euro

I have read that the fuel pump looks at the rpm and if its less than 300 rpm, it will shut off. Since you are having tack problems, it could be reporting an incorrect rpm to the fuel pump, in fact it could be oscillating around 300 rpm very rapidly causing it to turn on and shut off rapidly cauing the buzzing you mentioned.

__________________
1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.

I have thought the tach could be causing some of these problems but a local mechanic I use said no and something about the engine controls getting the RPM data from the distributor (don’t quote me on this one). I think I saw the CD manual showing the tach gets its data from the distributor. There is a plastic cap inside the distributor that is chipped around the edges. I wonder if something as simple as a piece of plastic (doubt it) or even a bad sending unit for the RPM’s could cause problems. I really don’t know how the tach or fuel pump relay gets that signal.

Steve
85 500SL Euro