FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A pilot shuts down a fuel-injected engine after a long taxi in hot weather and, upon attempting a restart minutes later, experiences difficulty starting with erratic or no fuel flow indications. This condition is most likely caused by:
- AVapor lock caused by fuel boiling in the lines due to residual engine heat
- BA stuck float in the carburetor bowl
- CDetonation occurring in the fuel lines
- DIce forming in the induction system
Show answer & explanationAnswer & explanation
Correct answer: A. Vapor lock caused by fuel boiling in the lines due to residual engine heat
Vapor lock occurs when fuel in the lines is heated (often by residual engine and fuel pump heat after shutdown) enough to vaporize, forming vapor bubbles that block or restrict liquid fuel flow. This is common with fuel-injected engines after shutdown in hot conditions, causing hard restarts or erratic fuel flow.
Why the other options are wrong
- B. A stuck float applies to float-type carburetors, not fuel-injected systems.
- C. Detonation is an abnormal combustion event inside the cylinder, not a fuel line phenomenon.
- D. Induction icing requires moist air conditions and engine operation, not a hot-soak restart scenario.
Vapor Lock
A condition where fuel in the lines vaporizes due to heat, forming vapor bubbles that block or restrict fuel flow, commonly occurring in fuel-injected engines after hot shutdowns.
- Caused by residual engine/fuel pump heat after shutdown
- Results in hard starting or erratic fuel flow
- Mitigated by boost pump operation or cooling period before restart
Memory trick: Vapor lock = fuel boils in the pipe and blocks the flow like a traffic jam of bubbles.