Yanmar diesel engines use a high-pressure, direct injection fuel delivery system. Fuel is drawn into the system with the lift pump. The high-pressure fuel injector pump then pressurises the fuel and delivers it, in precisely metered bursts, to the injectors through the fuel oil piping. Any air in the fuel system can lead to an erratic or non-running condition. Air should be bled from the fuel system whenever the fuel oil filter is changed or any of the fuel oil pipe fittings are loosened. The fuel oil filter must be primed with diesel fuel during the filter change in order for the lift pump to be able to force fuel through the system and bleed the air out.
Turn the fuel control valve to the "On" position. You must have fuel in the tank before beginning this process.
Loosen the air bleed screw on the top of the fuel filter canister with a screwdriver. Actuate the manual lift pump lever until fuel is expelled from the bleeder. Pump the fuel until the fuel escaping the bleeder is free of air bubbles. Tighten the bleed screw down firmly.
Loosen the fuel return pipe bolt with a wrench at the last injector before the pipe re-enters the fuel pump. Actuate the manual lift pump lever until the fuel coming from the pipe is free of air bubbles. Tighten the fuel return pipe bolt securely with a wrench.
Loosen the air vent screw at the injector pump with a screwdriver. Actuate the manual lift pump lever until the fuel coming from the pipe is free of air bubbles. Actuate the manual lift pump lever until the fuel coming from the vent is free of air bubbles. Tighten the vent screw securely with a screwdriver.
Loosen the nipples at both ends of the injector pipes with a wrench. Loosen the fuel delivery valve on the injector pump with a wrench. Turn the engine over with the starter until fuel comes out at the delivery valve with no air in it. Tighten the fuel deliver valve with a wrench. Turn the engine over with the starter until fuel comes from the injector pipe fittings with no air in it. Tighten the injector pipe fittings securely with a wrench.
Having oil absorbent (oil-sorb) pads on hand will aid in controlling and cleaning up the diesel fuel that is expelled during this process. The pad will absorb the fuel, but not water, and aid in removing any fuel that drips down into the engine room bilges.