Bleeding can be a good idea; in fact it can be a necessity when dealing with a diesel engine's fuel system. In this instance what we need to "bleed" is air that may have accumulated within a diesel engine's fuel system. The presence of air in a fuel filter, a fuel line or in the on-engine fuel system, including the high pressure fuel injection lines, can prevent a diesel engine from starting, make it run poorly or prevent it from developing full power. So getting the air out of the lines is crucial.
An automatic fuel bleeding system developed by Waterway Power Center, in Annapolis Md., for a military application that uses modest size diesel engines is now available for use with Nanni Diesel (Kubota) marine engines in the 10 to 130 horsepower range. (The system will likely work with most other mechanically fuel injected engines up to about 200 hp).
Designed by Klaus Hain, Jr., Senior Engineer at Waterway Power Center, the system is commendably simple; comprised of an electric fuel pump, a solenoid operated fuel flow control valve and an adjustable solid-state timer. The system's electrical circuit integrates with the engine control (ignition) switch used with Nanni Kubota diesel engines. The system provides two operating modes, automatic pre-start elimination of air in the fuel system and a timed fuel recirculating mode for use in removing contamination from fuel stored in the boat's tank when the engine is not in use.
The automatic pre-start mode is activated when the engine control switch is moved to the "preheat" or glow plug position, activating the bleed system's timer unit which applies power to the system's electric fuel pump and to the fuel bypass solenoid valve. The pump, connected to the outlet of the fuel system's pre-filter (typically a Racor water separating filter unit) draws fuel from the tank through the filter. The pressurized fuel is delivered simultaneously to the engine's fuel lift pump and to the energized solenoid valve. A substantial portion of the fuel and any air that was trapped in the primary fuel filter or fuel lines, for example as a result of changing the primary filter element, flows back to the fuel tank through the activated solenoid valve. A flow restriction built into the valve ensures that a sufficient flow of fuel is available to the engine to support full power operation even when the valve is open.
The system timer is adjusted on installation to provide the electric pump operating time needed to match the specifics of the boat's fuel system. Engine cranking begins when the glow plug has reached operating temperature with the electric priming pump operating. Any air that might be trapped in the fuel system beyond the engine's lift pump as a result of maintenance, or if the engine ran out of fuel, is scavenged and forced back to the fuel tank through the fuel return line. The pre-start mode is typically set to about one minute, although the timer can be set to any value up to 60 minutes. The time required for the system to purge any trapped air depends on the length and the size of the fuel system lines between the fuel tank and the engine. At the completion of the pre-start time, the self-bleeding system's timer turns off the electric fuel pump and removes power from the solenoid fuel bypass valve. The unpowered electric pump allows an unimpeded flow of fuel to the engine's fuel lift pump.
The self-bleeding fuel system's secondary operating mode uses the electric fuel pump to circulate fuel through the primary fuel filter when the engine is not in use. In this mode the system timer is changed from its normal, brief, pre-start time to its maximum, one-hour setting. The electric fuel pump moves a substantial amount of fuel through the primary fuel filter, returning the cleaned fuel to the tank via the activated solenoid valve. The sediment/water bowl at the bottom of the primary filter should be inspected and drained if there is any indication of water. At the completion of the fuel cleaning process, the system timer is reset to the time required for the pre-start mode.
In the event of failure of the engine-mounted lift pump turning on, the fuel bleed system will activate the electric pump, and though the solenoid operated fuel bypass valve will be open, sufficient fuel is supplied to the engine to allow full power operation.