FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
A mechanic inspecting continuous-flow fuel injection nozzles finds heavy carbon deposits partially blocking the nozzle orifices, causing rough idle. What is the most likely cause of this carbon buildup?
- AResidual fuel vaporizing and coking in the nozzle after a hot engine shutdown
- BUse of an incorrect nozzle size for the cylinder
- COverly rich fuel-air mixture during cruise flight
- DWater contamination in the fuel system
Show answer & explanationAnswer & explanation
Correct answer: A. Residual fuel vaporizing and coking in the nozzle after a hot engine shutdown
After a hot engine shutdown, residual fuel remaining in the nozzle passages absorbs heat from the cylinder and can vaporize/boil, leaving carbon (coke) deposits that restrict the small nozzle orifice, a common cause of injector nozzle fouling.
Why the other options are wrong
- B. An incorrect nozzle size would cause flow rate issues, not carbon deposits.
- C. A rich mixture may cause spark plug fouling but is not the typical cause of nozzle coking.
- D. Water contamination causes corrosion or icing issues, not carbon fouling.
Fuel Injector Nozzle Coking
Carbon deposits in continuous-flow injector nozzles typically result from residual fuel boiling off due to heat soak after a hot engine shutdown, restricting the nozzle orifice.
- Common in fuel-injected reciprocating engines after hot shutdowns
- Restricts nozzle orifice, causing lean mixture and rough idle
- Inspected and cleaned per manufacturer procedures
- Air pressure test used to check nozzle flow pattern and calibration
Memory trick: 'Hot Soak, Nozzle Choke' — heat after shutdown cooks fuel into carbon.