ASE A1 Engine RepairCylinder Head and Valve TrainMedium
A technician is diagnosing an engine with a persistent 'popping' sound heard through the intake manifold, especially noticeable during acceleration. A cylinder balance test indicates a weak cylinder on the engine. Which of the following is the most likely cause of this intake manifold backfire?
- AIncorrect ignition timing (too advanced).
- BA burnt intake valve that is not sealing properly.
- CA restricted exhaust system.
- DExcessive valve spring pressure on an intake valve.
Show answer & explanationAnswer & explanation
Correct answer: B. A burnt intake valve that is not sealing properly.
A 'popping' sound or backfire through the intake manifold, particularly when accompanied by a weak cylinder, is a classic symptom of a burnt or leaking intake valve. This allows combustion pressure to escape back into the intake manifold during the compression or power stroke.
Why the other options are wrong
- A. Incorrect ignition timing (too advanced) would typically cause pre-ignition or detonation, which might result in engine knocking or pinging, but not usually an intake manifold backfire.
- C. A restricted exhaust system would cause general power loss and engine overheating, not an intake manifold backfire.
- D. Excessive valve spring pressure would primarily cause accelerated wear on valve train components, not an intake manifold backfire.
Intake Manifold Backfire
A 'popping' sound or explosion in the intake manifold, usually caused by an intake valve not sealing properly, allowing combustion gases to escape into the intake.
- Causes a 'popping' sound through the intake.
- Often indicates a leaking or burnt intake valve.
- Can be caused by incorrect valve timing.
- Results in reduced engine power.
Memory trick: Intake pop, valve's got a flop.