DFFT Fuel Systems 1 — Questions and Answers
Question 1: A vehicle cranks normally but will not start. The fuel injectors are pulsing but there is no spark. What system should be diagnosed?
- The ignition system, including coils, crank sensor, and spark plugs (Correct answer)
- The fuel pump pressure output
- The engine's compression ratio
- The intake air temperature sensor
Correct answer: The ignition system, including coils, crank sensor, and spark plugs
Confirmed injector pulse means the PCM is running and fuel delivery is operating; the missing component is spark, so the ignition system is the correct area to diagnose.
Question 2: A vehicle has poor performance and black smoke from the exhaust. What does this indicate about the fuel mixture?
- The engine is running excessively rich with too much fuel (Correct answer)
- The engine is running lean with insufficient fuel
- The catalytic converter is overheating
- The EGR valve is stuck open
Correct answer: The engine is running excessively rich with too much fuel
Black smoke from the exhaust is unburned fuel, which indicates an over-rich mixture where there is more fuel than the available air can combust.
Question 3: A fuel pressure test shows pressure within specification at rest but drops rapidly during cranking. What does this indicate?
- A weak fuel pump unable to maintain pressure under flow demand (Correct answer)
- A leaking fuel pressure regulator diaphragm
- A clogged fuel filter causing high restriction
- A faulty fuel injector leaking internally
Correct answer: A weak fuel pump unable to maintain pressure under flow demand
A pump that holds static pressure but drops when the engine demands flow indicates the pump's output capacity is insufficient, a classic sign of a failing fuel pump.
Question 4: A vehicle starts normally but stalls at idle once it reaches operating temperature. What should be tested?
- Idle air control valve operation and throttle body deposits (Correct answer)
- Fuel injector spray pattern
- Fuel tank venting system
- Cold start injector function
Correct answer: Idle air control valve operation and throttle body deposits
A faulty IAC valve or heavily carboned throttle body can cause the engine to stall at idle once the richer cold-start enrichment is removed at operating temperature.
Question 5: What is the purpose of the fuel pressure regulator in a port injection system?
- To maintain a constant pressure differential between the fuel rail and intake manifold (Correct answer)
- To prevent fuel from returning to the tank when the engine is off
- To regulate fuel pump speed based on engine load
- To filter contaminants from reaching the injectors
Correct answer: To maintain a constant pressure differential between the fuel rail and intake manifold
The regulator maintains constant fuel pressure relative to manifold vacuum so injector flow rate depends only on pulse width, not on varying manifold pressure.
Question 6: A vehicle hesitates during acceleration from a stop. Fuel pressure is correct and spark is good. What should be checked next?
- Injector duty cycle and volume output test (Correct answer)
- Battery voltage during cranking
- Oxygen sensor switching frequency
- EGR valve position at idle
Correct answer: Injector duty cycle and volume output test
If pressure and spark are confirmed good, the next step is to verify each injector is actually delivering the correct volume of fuel, as a partially clogged injector can cause hesitation under load.
DFFT Fuel Systems 1
A vehicle cranks normally but will not start.
The fuel injectors are pulsing but there is no spark.
What system should be diagnosed?