ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is diagnosing an engine with a persistent 'popping' sound heard through the intake manifold, particularly at idle. The engine also exhibits a rough idle and reduced power. A compression test shows good and even compression across all cylinders. Which of the following is the most likely cause of these symptoms?
- AA restricted exhaust manifold.
- BA leaking intake valve.
- CA collapsed hydraulic lifter on an exhaust valve.
- DA worn exhaust valve seat.
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking intake valve.
A 'popping' sound through the intake manifold indicates that combustion pressure is escaping back into the intake during the compression or power stroke. This is typically caused by an intake valve that is not sealing correctly or is opening too early, allowing pressure to escape into the intake system.
Why the other options are wrong
- A. A restricted exhaust manifold would cause general engine power loss and potentially a low vacuum reading, but not typically a popping sound from the intake.
- C. A collapsed hydraulic lifter on an exhaust valve would cause a ticking noise and reduced valve lift, leading to reduced exhaust flow and power loss, but not a popping sound through the intake.
- D. A worn exhaust valve seat would cause air to escape into the exhaust system (a 'hissing' sound from the exhaust), not the intake.
Intake Manifold Backfire
A condition where combustion gases or unburnt fuel ignite in the intake manifold, often caused by a leaking or improperly timed intake valve.
- Results in a 'popping' or 'backfire' sound from the intake.
- Indicates combustion pressure escaping into the intake.
- Commonly due to leaking intake valves or ignition timing issues.
Memory trick: Intake pop, valve's a flop.