Fuel Systems Flashcards

What type of signal does a flex-fuel ethanol content sensor typically send to the PCM?

Answer: A frequency of about 50–150 Hz

Flex-fuel sensors put out a frequency that rises with ethanol content, typically about 50 Hz for E0 to 150 Hz for E100.

Water contamination is suspected after a vehicle starts misfiring and hesitating right after a fill-up. What is the best first check?

Answer: Draw a fuel sample into a clear container and look for separation

Water is heavier than gasoline and settles in a visible layer in a clear sample.

What is the purpose of a fuel pump inertia switch used on some vehicles?

Answer: To shut off the fuel pump after an impact

The inertia switch opens the pump circuit in a collision to reduce fire risk and must be reset afterward.

With the engine running, battery voltage is 14 V but the fuel pump connector shows only 9.5 V. What does this indicate?

Answer: Excessive resistance in the pump power or ground circuit

A 4.5 V loss before the pump shows high resistance in the circuit, which slows the pump and cuts output.

Which symptom most strongly points to a restricted fuel filter?

Answer: Loss of power under heavy acceleration with a normal idle

A restriction limits flow most when fuel demand is high, so idle can seem normal.

At warm cruise, short-term fuel trim reads +5% and long-term fuel trim reads +15%. What do these readings show?

Answer: The PCM is adding about 20% fuel to correct a lean condition

Positive fuel trims add together, so the PCM is adding about 20% fuel to make up for a lean condition.

On a return-type system with a vacuum-referenced pressure regulator, what happens to fuel pressure when the regulator vacuum hose is removed at idle?

Answer: Pressure rises about 8–10 psi

Without manifold vacuum helping it open, the regulator holds higher pressure, about the same as at wide-open throttle.