Fuel Systems Flashcards
6 cards from real DFFT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Fuel Systems flashcards as text
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?
Answer: Fuel pressure retention when the engine is cold and injector flow
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.
A vehicle runs perfectly until the fuel tank drops below one-quarter full, then it hesitates and stumbles. What is the most likely cause?
Answer: A fuel pump that is failing and cannot maintain pressure when the tank level is low
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.
A vehicle with a returnless fuel system has high fuel pressure. What is the most likely cause?
Answer: A restricted fuel return line or a faulty pressure regulation strategy in the PCM
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.
What could cause an engine to run well at idle and light load but lose power and surge under heavy acceleration?
Answer: A clogged fuel filter causing high restriction that only becomes evident under high flow demand
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.
A scan tool shows long-term fuel trim at +22% on both banks of a V8 engine. What is the most likely cause?
Answer: A large vacuum leak or a faulty MAF sensor under-reading airflow
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.
What is the danger of running a vehicle on a near-empty fuel tank regularly?
Answer: It reduces fuel pump life by causing it to overheat and draw in air
The in-tank fuel pump uses fuel for cooling and lubrication; running the tank low causes the pump to run hot and draw in air, accelerating wear and shortening its service life.