Diesel Fuel System


 Ask Dave / Posted 3 years ago / 725 views

Question: How does my diesel’s  fuel system work and waht do I need to know about it?

Dave’s Answer: The most common engine problem area is the fuel system.

Motorhomes today give the joy of freedom to travel as they are pushed down the road by a big diesel engine. The engine, Caterpillar or Cummins, is the main power plant manufacturers have provided to owners.

Lets trace the fuel flow to the engine and it’s return to the fuel tank.

  • On most diesel motor homes there is a big fuel tank mounted up front, and an engine mounted in back. In between the front and back there are fuel tubes or lines, some models have 2.
  • On top of fuel tank is a pickup tube. Most fuel lines are made out of either rubber or hydraulic hose, some are plastic lines especially on later models.
  • The fuel hose extends to the back of the motorhome to the engine. In between the tank in front and the engine in back, there is a primary fuel filter, which may also be a water separator. The fuel lines exit the primary fuel filter and connect to the fuel transfer pump. This applies to a Caterpillar engine, other makes may be different.. The fuel then exits the fuel transfer pump under pressure to a secondary fuel filter. To prevent problems this must be a CAT 1R-0751 2 micron filter for 3126 to Cat C-9  engines. The bigger C-10 to C-15 engines require a CAT 1R-0749 filter.  
  • Leaving the  secondary filter, fuel enters the engine. At this point the fuel enters what we call a common fuel rail and flows to all injectors where it is injected into the cylinders for combustion. The unused fuel then exits the head at the rear of the engine into a fuel pressure regulator. The regulator retains fuel pressure within the engine and allows the remainder to return to fuel tank in the front.

This give you a basic idea of how the system works, we will get into some of the possible problems next. I recommend using only Caterpillar parts in the fuel system since the filters themselves help maintain the proper fuel pressure that the EMC uses to regulate the engine.  After market filters may have the same filtration ratings, but who knows if they maintain the proper pressure necessary to regulate the engine.


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Fuel System

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