Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairMedium
A vehicle owner complains that their vehicle occasionally stalls after refueling, especially if they 'top off' the tank. There are no 'Check Engine' lights illuminated. What is the MOST likely cause of this symptom?
- AA faulty fuel pump relay.
- BA malfunctioning exhaust gas recirculation (EGR) valve.
- CLiquid fuel entering and saturating the charcoal canister.
- DA restricted catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: C. Liquid fuel entering and saturating the charcoal canister.
Overfilling the fuel tank can force liquid fuel into the charcoal canister, which is designed to store fuel vapors. When the canister purges, it draws liquid fuel into the engine, causing it to run excessively rich and stall.
Why the other options are wrong
- A. A faulty fuel pump relay would cause general starting or running issues, not specifically after refueling and topping off.
- B. A malfunctioning EGR valve typically causes rough idle, misfires, or poor performance, but not specifically stalling after refueling.
- D. A restricted catalytic converter would cause a loss of power and overheating, not stalling specifically after refueling.
Canister Saturation (EVAP)
Charcoal canister saturation occurs when liquid fuel enters the canister, impairing its ability to store vapors and potentially causing engine stalling during purge cycles.
- Caused by 'topping off' the fuel tank.
- Liquid fuel in canister leads to rich condition.
- Symptoms: stalling, rough idle after refueling.
Memory trick: Topping off means liquid fuel in the canister, making the engine choke on rich fumes!