DFFT Fuel Systems 2 — Questions and Answers
Question 1: A vehicle has a strong fuel odor after parking. No obvious leaks are visible. What system should be inspected?
- The evaporative emission (EVAP) system for leaks (Correct answer)
- The fuel injector seals for weeping
- The exhaust gas recirculation system
- The positive crankcase ventilation valve
Correct answer: The evaporative emission (EVAP) system for leaks
Fuel vapor odor without visible liquid leakage is characteristic of an EVAP system leak, where fuel vapors escape from hoses, the charcoal canister, or the purge valve.
Question 2: A scan tool shows a P0441 EVAP incorrect purge flow code. What does this indicate?
- The EVAP purge valve is stuck closed or the vent line is blocked (Correct answer)
- The fuel tank pressure sensor has failed
- The charcoal canister is saturated with fuel
- The fuel filler cap seal is leaking
Correct answer: The EVAP purge valve is stuck closed or the vent line is blocked
P0441 indicates the PCM detected insufficient flow through the EVAP purge circuit, typically due to a stuck-closed purge solenoid or a blockage in the purge line.
Question 3: A diesel vehicle emits white smoke from the exhaust on startup that disappears when warm. What is the most likely cause?
- Unburned diesel fuel from cylinders with a late injection timing or cold combustion (Correct answer)
- Engine coolant entering the combustion chamber
- A cracked diesel particulate filter
- Contaminated diesel fuel with high water content
Correct answer: Unburned diesel fuel from cylinders with a late injection timing or cold combustion
White smoke on a cold diesel start is typically unburned fuel caused by cold cylinder walls preventing complete combustion; it clears once the engine warms and combustion efficiency improves.
Question 4: What does a scan tool reading of 0% on a fuel trim at wide open throttle (WOT) indicate?
- The ECU has disabled fuel trim corrections at WOT, running from the base fuel map (Correct answer)
- The engine is perfectly tuned with no correction needed
- The oxygen sensor has failed at high load
- The fuel pump is unable to supply enough fuel under load
Correct answer: The ECU has disabled fuel trim corrections at WOT, running from the base fuel map
Most ECUs disable closed-loop fuel trim at WOT and use open-loop fueling from a base calibration map, which is why trim reads 0% or is not reported at full throttle.
Question 5: A vehicle's fuel economy has dropped significantly without any obvious drivability issues. What should be checked?
- Oxygen sensor response time and catalytic converter efficiency (Correct answer)
- Fuel pump delivery pressure at idle
- Throttle position sensor sweep linearity
- Coolant temperature sensor voltage at operating temperature
Correct answer: Oxygen sensor response time and catalytic converter efficiency
A lazy oxygen sensor or inefficient catalytic converter can cause the ECU to run slightly rich without triggering drivability complaints, but consistently worse fuel economy.
Question 6: A high-pressure direct injection engine misfires intermittently under load. The low-pressure pump is fine. What should be tested?
- The high-pressure fuel pump and fuel pressure at the injector rail (Correct answer)
- The intake camshaft variable valve timing actuator
- The ignition coil output on the misfiring cylinder
- The EGR valve pintle position sensor
Correct answer: The high-pressure fuel pump and fuel pressure at the injector rail
Direct injection engines use a high-pressure pump driven by the camshaft; if this pump cannot build sufficient rail pressure under load, injectors cannot atomize fuel properly and misfires occur.
DFFT Fuel Systems 2
A vehicle has a strong fuel odor after parking.
No obvious leaks are visible.
What system should be inspected?