FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
A pilot attempts to restart a fuel-injected engine shortly after shutdown on a hot day and finds the engine difficult to start, with erratic backfiring. What is the most likely cause, and what technique should the pilot use?
- ACarburetor ice has formed; apply full carburetor heat before restart
- BThe mixture is too lean at idle; enrich the mixture and reduce throttle to idle
- CThe magnetos have failed; switch to the alternate ignition system
- DFuel vapor has formed in the fuel lines from residual engine heat; use the auxiliary fuel pump on high with the mixture rich and throttle open to purge vapor
Show answer & explanationAnswer & explanation
Correct answer: D. Fuel vapor has formed in the fuel lines from residual engine heat; use the auxiliary fuel pump on high with the mixture rich and throttle open to purge vapor
Fuel-injected engines are prone to vapor lock after shutdown because residual heat vaporizes fuel in the lines, causing hard hot starts. The recommended technique is to run the auxiliary (boost) fuel pump on high with the mixture rich and throttle slightly open to purge vapor and re-establish a solid fuel flow before engaging the starter.
Why the other options are wrong
- A. Fuel-injected engines don't use carburetors, so carb ice is not applicable.
- B. A lean mixture alone doesn't explain hot-start vapor issues in fuel-injected engines.
- C. Dual magneto systems rarely both fail simultaneously and would not explain this symptom pattern.
Vapor Lock (Fuel Injection)
A condition where residual engine heat vaporizes fuel in injector lines after shutdown, causing hard starting.
- Common in fuel-injected engines after hot shutdown
- Corrected with aux fuel pump on high, mixture rich, throttle cracked
- Not related to carburetor icing
Memory trick: 'Hot fuel turns to bubbles—purge with the pump'