Greetings, I am having a problem with my 300SDL, occasionally and becoming more frequent, while at idle the rpm jumps by 500 rpm, it will then go back to normal. its like a sudden surge in power. The whole car kind of jumps up and it feels like the transmission has a surge as well. I have only experienced this surging below 1000 rpm. As of late the problem is increasing. Yesterday it happened three times while waiting at the bank. When stopped I am putting the car in neutral as a way to monitor the surge. Any input would be appreciated. My timing belt is in good condition.

Thanks

Two things I might suspect. The linkage to the injection pump is unstable. being the return spring is not holding the linkage in the idle position well. Probably unlikely.

Or something is erratic in the injection pump. I would flush soak the injection pump in place unless it appears to be some sort of oddball air issue. This is unlikely as well. Are you burning anything other than diesel fuel?

It can be other things but I might tend to look at the above things first. I seldom suspect the injection pump but still you cannot totally ignore it as a possibility sometimes. Welcome to the site.

It is your ELR kicking in. You can verify by pulling the plug halfway off the ELR and measuring voltage across the plug when you see the spike. Is your tach working? AC on or off? There is a decent troubleshooting section in the FSM, it may be in the w201 for the 602 or the w124 603, CRS.

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Is there any play in the throttle linkage?
Isn't there something that kicks in to raise the idle for when the A/C is on and the car at idle, to compensate for the extra draw from the compressor?

Is the idle smooth? Could it be the car is running on 5 cylinders and RPM increases when the 6th cylinder fires?

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Forgot about it being the sdl. Or 603 engine. I agree the electrical idle control is probably the issue. I would start by cleaning all the conections in that system well.

I agree, it's the EDS control system most likely. What Wimmutt is trying to tell you is to look at the idle control system on the injection pump.

On the injection pump there are two electrical connections.

One on the top (the sensor) and one on the rear (the actuator).

You want the connector on the rear of the IP. Pull it up about half way so that it is still making contact and put your meter probes on the exposed pins. If the voltage jumps up in time with the RPM surge there's your problem.

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2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
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The idle adjustable control may just have oxidized contacts for example. The heat in the engine compartment may be just enough to impact them for example. Cleaning connections and contacts would be my first line of approach.

Anyways in general this fault is probably a far better situation that what may have evolved with a previous type injection pump.