FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
During a ground run of a reciprocating engine equipped with a float-type carburetor, a mechanic notices that fuel pressure at the carburetor inlet rises well above the specified limit as engine RPM increases, and the mixture becomes excessively rich. Which defect in the engine-driven fuel pump is the most likely cause?
- AA cracked diaphragm allowing fuel to leak into the crankcase
- BA worn drive coupling reducing pump speed
- CA weak or broken relief valve spring allowing pressure to build unchecked
- DA clogged inlet screen starving the pump of fuel
Show answer & explanationAnswer & explanation
Correct answer: C. A weak or broken relief valve spring allowing pressure to build unchecked
The relief valve in an engine-driven fuel pump bypasses excess fuel back to the pump inlet to maintain a constant outlet pressure regardless of RPM. If the relief valve spring is weak or broken, it cannot open properly to bleed off excess pressure, so pressure rises with RPM and the mixture becomes overly rich.
Why the other options are wrong
- A. A cracked diaphragm typically causes fuel leakage and low pressure, not high pressure.
- B. A worn coupling would reduce pump output and pressure, not increase it.
- D. A clogged inlet screen would starve the pump, lowering pressure.
Engine-Driven Fuel Pump Relief Valve
A spring-loaded valve inside the fuel pump that routes excess fuel back to the pump inlet to hold outlet pressure constant despite changes in RPM.
- Maintains steady fuel pressure to carburetor
- Failure causes pressure to rise with RPM
- Excess pressure causes overly rich mixture
Memory trick: Relief valve is the pressure 'safety valve' bleeding off extra fuel like a pot's steam vent.