DFFT Fuel Systems 3

Question 1

A vehicle cranks but will not start in cold weather. The battery is fully charged. What should be checked on a fuel-injected gasoline engine?

Accepted Answer

Fuel pressure retention when the engine is cold and injector flow

Answer

Cold fuel has higher viscosity and cold cylinders need extra fuel enrichment; if the pump leaks down overnight or injectors flow poorly, cold starting becomes difficult.

Question 2

A vehicle runs perfectly until the fuel tank drops below one-quarter full, then it hesitates and stumbles. What is the most likely cause?

Accepted Answer

A fuel pump that is failing and cannot maintain pressure when the tank level is low

Answer

A failing fuel pump's motor windings depend on fuel for cooling; when the tank is low, the pump overheats and struggles to maintain pressure, causing drivability issues that recover when the tank is refilled.

Question 3

A vehicle with a returnless fuel system has high fuel pressure. What is the most likely cause?

Accepted Answer

A restricted fuel return line or a faulty pressure regulation strategy in the PCM

Answer

In a returnless system, pressure is controlled by the ECU varying pump speed or by a tank-mounted regulator; a restriction in the line or a faulty regulator causes abnormally high pressure.

Question 4

What could cause an engine to run well at idle and light load but lose power and surge under heavy acceleration?

Accepted Answer

A clogged fuel filter causing high restriction that only becomes evident under high flow demand

Answer

A partially clogged fuel filter can pass sufficient fuel at low demand but creates excessive restriction at high flow rates, starving the engine of fuel during heavy acceleration.

Question 5

A scan tool shows long-term fuel trim at +22% on both banks of a V8 engine. What is the most likely cause?

Accepted Answer

A large vacuum leak or a faulty MAF sensor under-reading airflow

Answer

High positive fuel trim on both banks simultaneously points to a global lean condition rather than a bank-specific issue; vacuum leaks or a faulty MAF sensor affect all cylinders equally.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Fuel Systems 3