Hey guys,

Betsie has had this issue ever since I bought it, but I never really got around to paying much attention to it.

The car idles and drives fine, but when I touch the accelerator pedal very lightly when the car is in neutral (to bring the idle RPM up just a little), the engine has this weird surging idle going on. It sounds like "whoomp whoomp whoomp" (about twice a second).

The simple answer would be: "don't press the pedal very lightly", but the engine also does this when the car is coasting at 10-15 mph. So any time I'm driving and I need to brake to come to a stop - or an almost stop - there is a short period of time where the idle does this weird "whoomp whoomp whoomp" thing.

Since it's hard to explain what the problem is, here is a short video:

https://youtu.be/3vrX2cZytBw

In the video I am putting very slight pressure on the accelerator pedal which makes it do its weird idle. I then rev it briefly twice and put a bit of pressure on the accelerator again, so it does its weird idle again.

Could anyone point me to a possible cause and fix? I'd have no idea where to start diagnosing this.

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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)

Probably, you could perform many maintenance items (valves, injectors, timing) and end up with a reduction in this behavior.

The greatest change from one item might come from adjusting the vertical screw on the engine side of the injection pump. -conventionally called "idle adjustment", that it isn't really accurate. that screw affects the return speed of the engine to idle and if it is raised too much will cause over fueling at idle. (look for a writeup by member: OM616, he did a great job explaining this)

Specs for idle say 750 +- 50 this isn't always the case and tachometers are often inaccurate. You just need to find that spot where the engine idles healthy, shifts well and always returns to idle when hot.

This behavior is often injection pump related. Some might say rack damper pin / bolt - that might be the case. But usually when there is some pressure on the linkages, this isn't true.

many injection pumps on these cars 'wear' into this behavior or have always been this way. best course of action is to keep up the related maintenance items then adjust your idle as needed.

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What? on a W123?

Actually, I am not sure of what the cause of the lope might be, although some on the Cummins Diesel Forums talk about this.
It seems quite distinct in the cabin, I would suggest checking the air cleaner mounts, air cleaner (how's your filter?) and exhaust system to see whether or not some sounds are being transmitted through them into the cabin.

I have not heard of this before in a W123, hopefully some members with this experience will chime in!

Good luck, and if the cause is determined please update this thread!

snapped_bolt

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'81 240D For now, a good place to borrow new parts
'80 300TD Probably will be put back into service!
'79 240D BACK IN SERVICE SINCE 09/16; limited use, oil leak. Guide pin r/sealed/replaced. Still a leak. Front crank seal....

The valves I adjusted about three weeks ago, so those are all in spec. (I did err on the side of a little too loose than too tight). I also did a diesel purge. Oil change and fuel filter, oil filter and air filter I replaced three weeks ago as well.

The state of the injectors so far is unknown. I haven't taken them out yet and they have never been pop-tested.

As far as the bolt on the injection pump goes: Do you mean the rack damper bolt?

I did adjust this with a hot engine and the car in Drive (my wife holding the brake) to where it just eliminates the rocking at idle, so I think that is pretty much set correctly.

The car idles absolutely fine when she's standing still and I don't touch the pedal; this weird surging/loping happens whenever the car is rolling at 10-15 mph (and so the gearbox is driving the engine idle up just a bit) or - like in the video - when I press the accelerator pedal ever so slightly.

Could this be injector related? I do have some nailing for about 40 seconds with ice-cold starts, so I know that will require some work, but I wouldn't think that could cause this strange idle.

__________________
"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)

Not the horizontal bolt on the back of the IP (rack damper)

there is a vertical screw that has a spring pin that the linkage is pushed against when you manually shut the car down in the engine bay. That is the one I am talking about. This will effect governor spring tension. (errr something like that)

Injectors, when well balanced, will help remove rhythmic disruptions (harmonics) that might find their way to the injection pump and also disrupt the governor.

Also, make sure the IP is well mounted to the engine. 3 bolts on the flange and one in the rear underneath near the oil filter housing.

There should be a return spring back there also on the rear mount.

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Sounds like something is rubbing. Per video, the car is shaking at the same frequency as the whoomf whoomf. Is that true? Does it make the same sound in gear (parking brakes on, wheels chocked)? Repeat the test with all the belts off and see if anything changes. Warm the engine up first then remove belts and run no more than 15 seconds since water pump is not turning.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

@Jt20
Thanks! I'll go take a peek-a-boo

@Funola
Yes, the car shakes at the same frequency.

Again, there is only a very specific "sweet spot" where this happens. When the car idles, she's fine. If I touch the accelerator ever so slightly she does this weird whoomf whoomf thing, and if I press the accelerator slightly further she has a nice (slightly) elevated idle run.

So during normal driving you never get this weird "whoomf whoomf" idle, except when coasting at a slow speed (10 Mph) or when coming to a complete stop. Because, when you come to a complete stop and as the car is approaching 10-15mph the idle does this weird whoomf whoomf thing, until you come to a complete stop, and then she idles normally.

She does the same thing when in gear. It seems to be related to engine RPM, where when the RPM are just a little above idle this happens, and when RPM goes up a little higher still it disappears again. It's weird...:|

__________________
"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)

good Luck.

I spent days chasing this issue on one car that had nearly new injectors and a very healthy engine. using a mixture of rack bolt adjustments and the vertical "idle screw" will help way more than injectors, I think. The next thing I would do is the injection timing.


make sure all the linkages arent bouncing around and all the springs are in good shape. try to recreate the issue with your head in the engine bay to get a better look.

And, for the record, I misspoke in that first post. You turn the vertical screw IN to increase idle. (I was speaking of 'turning idle UP") sorry.

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Does your car have the OE-style cigar-shaped hose running from the spin-on filter to the return line? (Sorry, I'm at work and videos are blocked here, so I can't see.)

If not, try getting the "correct" cigar hose and installing it.

It is precisely this behavior that the cigar-style hose is intended to correct.

Also an air leak into the fuel system can produce this behavior as well. Had it with my old w116 300sd.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Interesting. I need to take a peek, but I think it might have just a straight hose there. I'll have to take a peek tonight. Thanks!

__________________
"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)