How do you bleed air from the fuel system on a Yanmar marine diesel engine?
I recently changed the fuel filters on my Yanmar 3YM30 and now it will not start. It cranks over fine but refuses to fire, so I am sure air has gotten into the fuel lines. How exactly do I bleed the air out of the system?
Answer
To bleed a Yanmar diesel, you must purge air sequentially from the primary filter, the engine-mounted secondary filter, the high-pressure injection pump and the injectors. Start at the tank end and work towards the engine using the manual lift pump lever to force fuel and trapped air out of the bleed screws.
Bleeding the primary fuel filter
Keep absorbent pads handy and ventilate the space to avoid diesel fumes and fire hazards.
Always start at the primary filter mounted on the engine room bulkhead. For a top-loading Racor 500 series, fill the bowl to the brim with clean diesel before replacing the lid. For a CAV or Delphi filter, open the 10mm bleed screw on the top of the housing by two turns. Gravity pushes the air out if the fuel tank sits higher than the filter. If the tank sits lower, use the manual lift pump on the engine to draw fuel through. Close the screw firmly once a steady stream of diesel flows out without bubbles.
Purging the secondary engine filter
Move to the secondary fuel filter mounted directly on the Yanmar engine block. Locate the mechanical lift pump. You will usually find this low down on the starboard side on models like the 3YM30 and 3GM30. It features a small metal priming lever.
Open the 10mm bleed screw on top of the secondary filter housing. Pump the lift pump lever up and down. If the lever feels slack and offers no resistance, the internal camshaft lobe is holding the pump diaphragm open. Briefly flick the engine starter to rotate the camshaft. This releases the pump and restores a full pumping stroke.
Keep pumping until solid fuel weeps from the bleed screw, then nip it shut. Do not overtighten this fixing. A torque of 8 to 10 Nm is plenty, as the hollow bolts snap easily.
Bleeding the injection pump
Follow the fuel line from the secondary filter to the high-pressure injection pump. Look for the small bleed screw on the pump body. This is often a 10mm hex or Phillips head. Open it a turn or two and pump the lift pump lever again until clear fuel flows. Nip the screw shut.