3208 – SMFS Pressure Relief To Tank

SMFS Pressure Relief To Tank
(1) Pressure relief to tank valve. (2) Spring.

Some sleeve metering fuel systems feature a pressure relief to tank valve (1) and spring (2) located under the torque control cover. The purpose of this feature is to maintain cooler gallery fuel by avoiding the fuel heating associated with recirculating excess fuel continuously back to the transfer pump inlet. Instead all excess gallery fuel is returned to the fuel tank. The return to tank flow also sweeps wear particles out of the fuel gallery. This configuration has the pressure relief to tank valve (1) located in an upper counterbore of the timing pin hole. A syphon break passage connects the gallery to the pressure relief passage. One half of the return to tank fuel leaves the gallery through the syphon break passage; the rest passes through the camshaft journal bearing to enter the bottom of the timing pin hole and pressure relief valve. An overpressure vent feature has been added to this configuration to protect the SMFS from failure in the unlikely event that the return to tank line should become blocked. This overpressure vent is located in the fuel inlet flange. The check valve used to bypass the transfer pump during priming pump gallery fill has been mounted on a nylon seat. The valve is held against the seat by a spring. Normally the check valve allows flow only into the gallery. In the event of gallery overpressure the check valve assembly lifts from its seat to prevent fuel back to the transfer pump inlet supply. The pump will function at increased gallery pressure to allow emergency operation, however, lip seal overload will greatly shorten the life of the seals. In addition the pump will overheat. Excess fuel pressure readings should be investigated to prevent premature fuel injection pump failure.

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