DFFT Fuel Systems 2

Question 1

A vehicle has a strong fuel odor after parking. No obvious leaks are visible. What system should be inspected?

Accepted Answer

The evaporative emission (EVAP) system for leaks

Answer

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?

Accepted Answer

The EVAP purge valve is stuck closed or the vent line is blocked

Answer

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?

Accepted Answer

Unburned diesel fuel from cylinders with a late injection timing or cold combustion

Answer

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?

Accepted Answer

The ECU has disabled fuel trim corrections at WOT, running from the base fuel map

Answer

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?

Accepted Answer

Oxygen sensor response time and catalytic converter efficiency

Answer

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.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Fuel Systems 2