My 91 300D just developed mild "kangarooing" upon hard acceleration. By kangarooing I mean an rapidly oscillating speeding up then hesitating.

Any ideas as to how to investigate?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

These cars are getting on now .What they could do when they were new, is not going to happen now ..old cars need treating that way,, or get the wallet out, and spend on failed items ..so i would get a good tune up first before throttling the engine to death ..If it was a horse it would die half way in to the journey ,,but its up to you .

Sounds like fuel starvation.

Change the fuel filter and if it still does it then start looking for air in the fuel lines possibly caused by tank screen getting blocked or just air ingress somewhere along the lines and joints in the system.
A blocked fuel filter will give the kangarooing but the tell tale here is to run it at a constant engine speed until it does it and then back off about 300 revs and it will behave itself and act normal until you increase the revs again. Another symptom of fuel starvation is white / grey smoke accompanying the surging ie not enough fuel to burn.
I have had a few partially blocked filters over the years because I was using veg oil and each time the symptoms were first spotted when on a long drive and using say 3k revs for over 20 minutes or more and then you pick up on the gentle slow up as it starts to starve of fuel so back off and it goes away and increase slowly and back it comes.
I found the om603 would tolerate a bad filter where the om606 would happily start drawing air in everywhere with its o ring fittings .

Isn’t that the truth? Rube Goldberg would have been proud of the OM606 fuel system.

But yeah Shertex I think you have a fuel problem.

I would check the fuel system for air ingress. You’re up north, make sure the lines under the car haven’t developed a rust through leak.

Also make sure the rubber hoses bridging between the engine and the body are good.

I would look at the fuel preheater thermostat also, that thing gets leaky. 10 years ago they had a rebuild kit for it, not sure if it’s still available.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
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)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks, guys. I will replace fuel filter and then proceed from there. Fuel filter was replaced about 2000 miles ago but I guess they can get clogged quickly. Fuel thermostat was replaced a few years ago.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Check your transmission fuel level. I remember i had a weird issue years ago when mine was a little low on fluid. Can not remember it exactly it was so long ago.

Not suggesting in the least this is your problem. Still check it properly on general principals. Takes no time.

It was the main fuel filter on the 300SD when we had a similar issue. Changing the lift pump filter didn't help but the main filter made a huge difference.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

A wild guess, but your input to the injection pump is really a "speed setpoint" request. If something is non-ideal inside it could suffer negative feedback oscillations, which James Watt dealt with in his 1800's steam engine regulator. That lead to an entire theory of control systems.

ATF level fine....replaced main filter. Inline filter looks clean. Issue persists. Now on to other possibilities.

Minor annoyance...I'd hate to throw much money at it. If I have a craving for acceleration, I have my pair of CDIs!

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Your fuel system has 3 filters, the tank strainer, the pre filter and the main filter.

My old e300d was slow as mud uptil the time I cleaned the tank strainer - then it had all its pulling power.

To the best of my knowledge my tank strainer has never been cleaned. Maybe that should be next on principle.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Disconnect exterior fuel tank hose. If fuel flows fine by gravity. One could assume the fuel tank filter is not obstructed.

Years ago we had a vehicle in that somehow their seemed to be some bunker C in the a gas tank. Periodically it would cover the tank filter it seemed.

Also make sure it is not a vacuum developing in the tank by leaving the fuel cover loose. Both are pretty simple to eliminate as potential causes. Not that they are the cause.

Both are actually unlikely yet the tests again are easy. Pulling out a tank filter you can prove is probably quite serviceable otherwise is a messy issue. Plus perhaps more work than is needed.

Personally i would run a hose back inside the car to monitor the fuel pressure in the base of the injection pump. Is it staying up or far too low. Another quick attempt to eliminate the injection pump perhaps. Is to close off the return line from the injection pump. If there is no change at all with that line closed off. You do have some issue in the fuel supply system. If it is enough to cause the surging is another thing.

Generally speaking injection pump failures of this type must be considered pretty rare on these old cars. Not impossible yet rare in my opinion.

The issue seems to have resolved by itself...weird.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Well just to be sure give it at least half a bottle of Startron starbrite diesel enzyme treatment in a full tank and use the rest on another.
Any nasties lurking will start to get sorted out .
The line out of tank obviously exits the bottom , but where it exits there is a small bell shaped plastic affair about the size of a coffee mug and this sits over the top of the tank strainer… the return line puts fuel back into this as well so any hot fuel goes straight back to engine.
Now if any crusty stuff is in there it sits on the strainer so what some owners do is swap return and fuel in line over up in the engine compartment where the rubber lines join the underbody steel ones .. this can be used as a get you by and works quite well but keep an eye on the pre filter because if anything is floating around in this bell some of it will end up getting caught altering you to what’s in there.
Removing the tank sender unit and having a peek inside the tank and the general state of the sender unit will tell you a lot. Use a mobile phone and torch and make a short video clip to see what’s happening in there.