5 Common Causes of Low Diesel Engine RPMs

Low max RPMs is a common symptom of propulsion problems. In full throttle, the tachometer does not go to the engine’s rated max RPM’s. Please read on for possible problems and troubleshooting options.

  1. Over propped – A simple problem is too large a prop. Often people put on extra large props for fuel economy. Test if in neutral aka “no load,” she can come up to speed. The tachometer should show 10% greater than the max RPM’s. Be careful though. A smaller prop may resolve the symptom, but the issue will remain if another factor is at play.
  2. Wrong Gear Box – If the transmission has been changed, you should double check if the reduction gear ratio changed. Any prop shop can tell you based on the prop and engine max RPMs, the corresponding gear ratio.
  3. Growth on Shaft – In the same way a large prop will overload the engine, too much growth of barnacles and algae will overload the engine. Get a diver down to clean the shaft.
  4. Tachometer – Maybe she is just right, and the tach is wrong? What a relief.
  5. Fuel System – Finally she may not be overloaded, but instead not getting the right ingredients for combustion. A blockage in the air intake, the fuel line, bad injectors, or a frozen governor could lead to such issues. Get a diesel mechanic to overhaul the fuel system.

This problem can be as simple as a new tachometer or as expensive as an engine overhaul. Please let me know any other thoughts in the comments below.

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