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Hey Engine Man: Why Does My Diesel Lack Power at High RPM?

Successful Farming Engine Man Ray Bohacz has engine grease and field dirt under his fingernails from a life spent repairing vehicles and running a farm. When he's not busy in the shop, he's working on maintenance articles and videos for Successful Farming magazine and answering questions from readers. The following is a letter Bohacz received from Steve Meyer:

My two-cylinder diesel tractor Belarus AS250 is having a problem that I am trying to diagnose. When running at low or medium RPM, the tractor seems to have full power. But when running at high RPM, the tractor experiences loss of power. What are some common causes for this occurrence?

Response from the Engine Man:

There are a few possible causes for a lack of power at high rpm with a diesel engine, but I like to first go to the most common causes. I am a firm believer in not looking to make a simple problem complicated.

I will list these in the most likely order:

  1. Starved for fuel: This may be caused by a restricted or a wrong fuel filter, air being introduced into fuel system via porous fuel line, loose connection, weak lift pump is so equipped, etc.
  2. Restricted air intake: This could be caused by a dirty, incorrect, or collapsing air filter inlet ducting.
  3. Advance mechanism stuck in injection pump.
  4. Vacuum being caused in fuel tank: To correct this, take the fuel cap off and see if there is a change in the operation of the engine.
  5. Old or very poor-quality diesel fuel.

Do you have a maintenance question? Email Ray Bohacz at

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