Fuel System - Overview C Series Page F-4 Section F - Familiarization Marine 480C-E engines use a Bosch® P7100 mechanical fuel pump with a Bosch® RE30 governor. A mechanical, positive displacement fuel lift pump provides pressurized fuel to the fuel filter head inlet. A keyswitch-activated electric fuel shutoff valve is mounted on the outlet of the fuel filter head. Filtered and pressurized fuel is then routed back to the fuel pump inlet. The purpose of the pressure relief system installed on the lift pump is to prevent over-pressurizeing of the fuel filter assembly upon shutdown.
When the engine is commanded to shut down, the keyswitch circuit is interrupted and causes the fuel shutoff valve to close. As the engine comes to a stop, fuel is still pumped to the fuel filter from the lift pump. It is possible that the pressure can climb high enough to cause the fuel filter seal or body to fail. The system uses a relief valve in the banjo connection screw located at the outlet of the lift pump. A hose is connected to the banjo to direct the fuel back to the lift pump inlet. The relief valve is a ball and spring type and is set to open at 690 kPa [100 psi] +/- 70 kPa [10 psi] and is not adjustable.
The relief valve should be open only after the engine has been commanded to stop. If the relief valve becomes stuck or does not seat completely, fuel will continuously be bypassed back to the lift pump inlet. This causes low lift pump output pressure and leads to poor performance and hard starting. Following the appropriate Troubleshooting Symptom charts in the Troubleshooting and Repair Manual will identify low lift pump pressure associated with a failed relief valve. NOTE:
MW-type fuel injection pumps without the fuel drain arrangement on the engine side will require additional venting prior to initial start-up, fuel injection pump replacement, or if the engine has been allowed to run out of fuel. Refer to Procedure 006-003 for more information.