Hi guys, I just saw this separate diesel discussion, so I will post this here, too- not just in the vintage section.

My name is Olaf, I am new here and live in South Africa.

I have recently got myself this old cheap, high mileage Diesel.

While being able handed and not entirely stupid, I have not been able to get the idling/revving of this car right.

The revs go up and down, undulating while idling AND while pushing on the throttle, also under load. It is not cutting out or knocking, just an irritating slight up and down of the revs.

I have so far:

-tried with/without clean oil bath air filter.

-deactivated/singled out potentially bad injectors (-not that it sounds like that, just to make sure and because I really didn't know what else to do.)

-bled the fuel system a couple of times and changed the fuel filters

-set the valves properly

-replaced the Diaphragm at the back of the injector pump(-the leather one!)

-replaced the vacuum hose that connects the IP to the intake.

I can sometimes "counter" the problem by carefully going against it with the pedal or playing with the idle adjuster but on the whole it still persists.

I'd be grateful for an idea or three on what causes this problem.

Regards
Olaf

Never dealt with this issue myself. I'd check a few things:

- Test for vacuum leak in the diaphragm cover on the back of the pump. Could leak where the "tickler" shaft goes through. Could leak at the vacuum line fitting.

- Check that the vacuum port in the throttle body is clear.

- Check the lengths of the control rods, the one that comes over the engine from the throttle body, and the one that goes down to the "tickler" on the pump.

Welcome to the forums!

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- Greg -
1973 220D, The Prodigal Benz
1974 240D

how does the frequency of the oscillations change with engine speed (rpm)?


I would likely rebuild the lift pump and the check valve (overflow valve) at the back of the injection pump if my injectors were all good and regular maintenance was taken care of.

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Hello,

The oscillations stay the same with increasing engine speed.

Do you think this is a fuel supply issue rather than an air leak/vacuum one?

You might try a diesel purge. Does your engine have heat shields on the injectors? If so, you probably need to replace those if you didn't whenever you pulled injectors to try to isolate the problem.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Does your car do the same as mine does at around the 9 second mark in this video?:

https://www.youtube.com/watch?v=3vrX2cZytBw&feature=youtu.be

If so: I haven't been able to determine what's causing that so far either...

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

Hi there,

No- that is not what it sounds like.

With mine there is no stutter, there is no hesitation in the ignitions of the cylinders, it is the engine speed/revs undulating only.

Fatigued spring in governor or preload set too low?

Have you adjusted the idle according to the FSM yet?

Idle adjustment

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Ok- I think I might have got it!

The Linkage rod going onto the pump lever was way out. I have set it so the lever is up about 8mm at idling now and that seems to have cured the problem.

I still need to do a bit more cold/hot etc testing

On to the next little issues.

Thank you guys for participating!