Question 1

The filter is clear, tank has fuel, seacock is open. You suspect air has entered the fuel system after changing the fuel filter last week. How do you bleed the fuel system on a typical Yanmar auxiliary diesel?

Accepted Answer

Open the bleed screw on the fuel filter head; use the lift pump priming lever to pump fuel through until no bubbles; close the screw; repeat at the injection pump bleed screw; then crank with throttle 1/3 open Air in the fuel system (introduced by filter change, running dry, or cracked pipe) prevents diesel engines from firing. Bleed sequence: 1) Filter head bleed screw — pump lift pump lever until bubble-free; 2) Injection pump bleed screw if fitted; 3) Some engines self-bleed up to the injectors if cranked long enough. NEVER pour fuel into the air intake — risk of hydrostatic lock and fire. Most Yanmar auxiliaries self-bleed up to the HP pump with 30-60 seconds cranking.

Question 2

You turn the key. No cranking sound at all — complete silence. What is the most likely fault category and what do you check first?

Accepted Answer

Electrical: check battery voltage (battery monitor or multimeter); check main battery isolator switch is ON and set to the correct bank; check the engine battery is not flat; check the kill/stop control is in the RUN position No crank = electrical fault (unless the decompressor or stop control is engaged). Sequence: 1) Is the battery isolator ON and on the correct bank? 2) Is battery voltage above 12V (10.5V minimum for cranking)? 3) Is the kill/stop control fully in RUN? 4) Is the neutral safety switch in neutral? Do not proceed to fuel diagnosis until you have confirmed electricity is reaching the starter motor.

Question 3

The battery isolator has two banks (1 = House, 2 = Engine, BOTH). The switch is on BOTH. The voltage reads 11.8V. What does this tell you and what do you do?

Accepted Answer

11.8V is borderline for reliable starting — switch to Bank 2 (engine bank only) which may have more charge; reduce all electrical loads; try cranking on the dedicated engine bank A diesel starter motor needs high current (200–500A peak). 11.8V under no load may drop to 9–10V under cranking load on BOTH banks — below the starter's cut-out. Switch to the dedicated engine bank, turn off navigation instruments and VHF to reduce parasitic load, and crank in short bursts (10 seconds max) to avoid overheating the starter. If still no crank, check the starter solenoid wire and starter motor itself.