FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationHard
During an engine ground run, a mechanic notices that fuel flow to a fuel-injected engine drops and the engine surges intermittently on a hot day shortly after a fast taxi and shutdown/restart cycle. Vapor is suspected to have formed in the fuel line between the tank and the fuel pump. Which factor is the most likely primary cause of this vapor lock?
- AExcessive fuel pressure from the engine-driven fuel pump forcing vapor formation
- BThe fuel selector valve being positioned to the wrong tank
- CHigh fuel line temperature combined with reduced fuel pressure on the suction side of the pump, causing the fuel to boil
- DContaminated fuel with excess water content boiling at lower temperatures
Show answer & explanationAnswer & explanation
Correct answer: C. High fuel line temperature combined with reduced fuel pressure on the suction side of the pump, causing the fuel to boil
Vapor lock occurs when fuel boils within the fuel lines, typically caused by high temperatures (from residual engine heat after shutdown) combined with reduced pressure on the suction side of the fuel pump. This lowers the fuel's boiling point locally, allowing vapor bubbles to form and disrupt fuel flow, particularly noticeable after hot restarts.
Why the other options are wrong
- A. Excessive pump pressure would tend to suppress vapor formation, not cause it; low pressure areas are the concern.
- B. An incorrect fuel selector position would cause fuel starvation from an empty tank, not vapor lock.
- D. While water contamination is a fuel quality issue, it does not describe the classic vapor lock mechanism caused by heat and pressure.
Vapor Lock
A condition where fuel vaporizes within fuel lines due to high temperature and low pressure, disrupting fuel flow, commonly occurring during hot restarts after heat soak.
- Caused by heat soak plus low pressure on suction side of pump
- More common in hot weather and after shutdown of hot engine
- Can cause rough running, power loss, or engine stoppage
- Boost pumps can help by increasing pressure to suppress vapor formation
Memory trick: Hot soak plus low pressure lets fuel boil into bubbles that choke the line