Bleeding a diesel engine

All it takes are a few simple tools and you are ready to start bleeding your diesel engine

Much has been said about how forgiving diesels are, but one thing they won’t tolerate is air in the fuel system. Air can enter for a number of reasons – running out of fuel or servicing or replacement of components being common causes. Although some engines claim to be self-purging, most need outside intervention.

The exact sequence may vary from engine to engine, but the one described here is typical. It involves filling the system with fuel from tank to injectors, usually by manually operating the lift pump. Simple? Of course!

-- Ensure the tank has plenty of fuel in it and its shut-off valve is open. Then...


  •  Try the lift pump’s operating handle to see if it works fully. If it doesn’t work at all or seems limited, give the engine half a turn (to move the operating cam) and try again.
  • If the fuel is drawn from the bottom of the tank and the pre-filter is below the level of the fuel, crack open the bleed screw and it will fill by gravity. If not, keep the bleed screw closed – it’s time to use the lift pump and turn your attention to the fine filter.
  • Open the bleed screw on the fine filter and pump away until bubble-free fuel emerges. Don’t be astonished if this takes some time. When satisfied, nip up the bleed screw.

    This should be enough for most engines, but not for all, including the GM series Yanmars, Perkins, Thorneycrofts and the various Kubota derivatives.
  • For them, the next step is the injection pump. Loosen its bleed screw (some older rotary injection pumps have two bleed screws and you will have to do both, starting with the lower one) and rejoin your labours on the lift pump. Once the fuel runs clear, close the screw (or screws) and give the lift pump a few more strokes for good measure.
  • This really should be enough. Try and start the engine in the normal way. If you fail you will have to bleed the injectors. Since these are on the high pressure side of the injection pump, the manual lift pump won’t be up to the task, so you will need to use the engine.
  • Loosen all the injection pump nuts at the injectors a couple of turns.
  • Set the throttle to full ahead, out of gear, and turn the engine over with the ignition key. Limit yourself to no more than 15 second bursts to avoid burning out the starter motor.
  • First you will see the inevitable bubbles around the nuts, but these will become squirts. At which time, harden down the injectors nuts and … start the engine!

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