diesel power surges

I have a '97 E300 diesel with about 135k miles. When holding fairly constant speeds of between 35 and 50 mph (with or without cruise control) the engine
surges at about one surge per second. It does not happen at low speeds or at high speeds. And it does not always do it. At first I thought it might be a bad batch of diesel, but after 3 refills it still does it. Any suggestions?
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This is a non-turbo engine so turbo issues can be eliminated. That leaves a few possibilities. Restricted air flow - an obstruction in the engine's air intake or a dirty air filter. Transmission: torque converter lock-up feature is locking and releasing, locking and releasing raising and lowering the engine's speed. Something with the injection system, but not having knowledge of this engine I have to leave it at that obviously limp suggestion which could include a dirty fuel injector.
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T.G., your input into this sight is most appreciated. FYI, I had my engine checked and the fuel lines on the engine were beginning to crack and leak. Put a new set on and all is well. BTW, having logged over half a million miles on 300SD engines, I find the performance on the non turbo '97 more than adequate. I'd buy another.

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a 1997 diesel with out a blower what were they smoking???? who would buy one?
case
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On a 3-litre it wasn't considered top priority then. Remember there were sub-2-litre diesels on the old days, too. The rise and rise of diesel fuel has resulted in significant advances in diesel engines.
For example, I just drove a 1.9-l turbodiesel (on a small Alfa Romeo). Great acceleration and from inside you'd never know it was a diesel.
DAS
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For direct contact replace nospam with schmetterling
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