My 1991 300D hasn’t been used in a few months due to winter and salt. Took it for a 110 mile trip yesterday, ran beautifully (other than the tick at idle - lots of other threads on that).

I felt like under load, on the interstate at speed, sometimes I’d get a very mild, almost imperceptible pulsing or surging feeling. Not a huge feeling, very mild, no rpm or noise changes. But I could feel it. The car has plenty of power, and can cruise along all day at much higher speeds if pushed.

It could be a mild headwind. Sure. I don’t know what else it would be. No brakes were grabbing or overheated. Tires all at pressure slightly above placard.

What is the phenomena called if it feels like you’re surging under load? It’s maybe once or twice per second let’s say. What’s the cause? What’s the solution? Obviously I’ll be investigating more, but wanted to start the discussion.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

Do you have the vacuum wastegate or pressure wastegate? Check your boost pressure, if you're running a little high, you could be tripping the overboost circuit and dumping boost briefly. EGR flow can also give this surging/hunting effect. You didn't mention if you were using cruise control or not.

Not on cruise control.

Stock setup so computer controlled vacuum operated wastegate.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

I've heard it called kangarooing. Experienced it on my CDI after a tune under hard acceleration. Tuner finally resolved the issue.

Do a search for acceleration, kangarooing and AI will spit out a mouthful of potential causes.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 161k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 147k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 81k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife