ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is diagnosing an engine with a consistent 'popping' or 'backfiring' sound heard through the intake manifold, especially during acceleration. The engine also exhibits a rough idle and reduced power. Which of the following is the MOST likely cause?
- AA faulty ignition coil on one cylinder.
- BA restricted exhaust system.
- CA leaking intake valve.
- DAn overly rich fuel mixture.
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking intake valve.
A 'popping' or 'backfiring' sound through the intake manifold (an 'intake backfire') is typically caused by combustion occurring when the intake valve is still open. This is most commonly due to a leaking or burnt intake valve, which prevents it from sealing properly, allowing combustion pressure to escape back into the intake.
Why the other options are wrong
- A. A faulty ignition coil would cause a misfire, but typically not an intake backfire unless the timing was severely off or fuel was igniting in the intake due to other issues.
- B. A restricted exhaust system would cause a loss of power and potentially overheating, but not typically an intake backfire.
- D. An overly rich fuel mixture usually results in black smoke, poor fuel economy, and possibly a 'loping' idle, but generally not an intake backfire.
Intake Backfire (Valve Issue)
A 'popping' or 'backfiring' sound heard through the intake manifold, indicating that combustion is occurring when an intake valve is not fully closed, often due to a burnt or leaking valve.
- Caused by combustion gases entering intake manifold.
- Primary cause is a leaking or burnt intake valve.
- Accompanied by rough idle and reduced power.
Memory trick: Intake pop, valve's flop, combustion won't stop.