Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A vehicle owner reports a significant drop in fuel economy, a strong smell of gasoline, and a rough idle that smooths out at higher RPMs. Visual inspection reveals no external fuel leaks. Which of the following is the MOST likely cause?
- AA restricted exhaust system
- BA contaminated mass air flow (MAF) sensor
- CA faulty oxygen sensor
- DA leaking fuel pressure damper diaphragm
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking fuel pressure damper diaphragm
A leaking fuel pressure damper diaphragm can draw liquid fuel into the vacuum line connected to the intake manifold, leading to a rich condition, rough idle, and gasoline smell, especially noticeable when vacuum is high (idle).
Why the other options are wrong
- A. A restricted exhaust system would cause power loss and potentially overheating, but not typically a strong gasoline smell or rough idle that smooths out due to a rich condition.
- B. A contaminated MAF sensor could cause a rich or lean condition, affecting fuel economy and idle, but is less likely to produce a raw gasoline smell without a physical leak.
- C. A faulty oxygen sensor could cause poor fuel economy, but typically wouldn't cause a strong raw gasoline smell unless it was causing an extreme rich condition, and it's less direct than a physical fuel leak into the manifold.
Fuel Pressure Damper Diaphragm Leak
A leak in the fuel pressure damper diaphragm allows raw fuel to be drawn into the intake manifold through the vacuum line, causing a rich condition.
- Symptoms include strong gasoline smell, rough idle (especially vacuum-related), and poor fuel economy.
- Often mistaken for a leaking injector or external leak.
- Diagnostic involves checking for fuel in the damper's vacuum line.
Memory trick: Smell of gas, rough idle mess, damper's stress, intake's excess.