DFFT Fuel Systems

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?

Accepted Answer

The ignition system, including coils, crank sensor, and spark plugs

Answer

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?

Accepted Answer

The engine is running excessively rich with too much fuel

Answer

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?

Accepted Answer

A weak fuel pump unable to maintain pressure under flow demand

Answer

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?

Accepted Answer

Idle air control valve operation and throttle body deposits

Answer

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?

Accepted Answer

To maintain a constant pressure differential between the fuel rail and intake manifold

Answer

The regulator maintains constant fuel pressure relative to manifold vacuum so injector flow rate depends only on pulse width, not on varying manifold pressure.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Fuel Systems