NAPA Engine Performance 5 — Questions and Answers
Question 1: Fuel pressure is in spec at idle but drops well below spec under hard acceleration. What is the most likely cause?
- Leaking injector
- Restricted fuel filter or weak pump (Correct answer)
- Faulty pressure regulator stuck closed
- Bad MAP sensor
Correct answer: Restricted fuel filter or weak pump
A restricted filter or weak pump cannot keep up with high fuel demand even though idle pressure looks normal.
Question 2: After shutdown, fuel pressure bleeds off quickly and the engine is hard to start when hot. Which cause is most likely?
- Leaking injector or faulty check valve (Correct answer)
- Clogged air filter
- Faulty TPS
- Weak ignition coil
Correct answer: Leaking injector or faulty check valve
A leaking injector or a failed pump check valve lets residual pressure drop, causing long cranks and hot-start trouble.
Question 3: An injector balance test shows one injector with a much smaller pressure drop than the others. What does this indicate?
- The injector is leaking
- The injector is restricted or clogged (Correct answer)
- The injector is flowing too much fuel
- The fuel pump is weak
Correct answer: The injector is restricted or clogged
A smaller pressure drop means less fuel passed through that injector, which points to a restriction.
Question 4: A contaminated MAF sensor typically causes what fuel trim pattern?
- Negative trims at all speeds
- Positive (lean) trims, especially at higher airflow (Correct answer)
- No change in fuel trims
- Rich trims only at idle
Correct answer: Positive (lean) trims, especially at higher airflow
A dirty MAF under-reports airflow, so the PCM adds fuel through positive trims, often most noticeably at higher loads.
Question 5: Which condition causes the PCM to run in open loop?
- Warm engine at steady cruise
- Cold engine before O2 sensors reach operating temperature (Correct answer)
- Idle with a warm engine
- Light acceleration at operating temperature
Correct answer: Cold engine before O2 sensors reach operating temperature
The PCM stays in open loop until the O2 sensors and engine are warm enough to give reliable feedback.
Question 6: A variable valve timing (VVT) code appears after an oil change with the wrong oil viscosity. Why?
- VVT phasers are controlled by oil pressure through a solenoid (Correct answer)
- VVT uses a separate hydraulic fluid
- Oil viscosity affects the knock sensor
- The O2 sensor reads oil vapor
Correct answer: VVT phasers are controlled by oil pressure through a solenoid
VVT phasers are moved by engine oil routed through the oil control solenoid, so wrong viscosity or low oil affects their response.
Question 7: A drive-by-wire throttle body is cleaned and the engine now idles erratically. What step is often required?
- Replace the throttle position sensor
- Perform a throttle body idle relearn procedure (Correct answer)
- Reset the timing belt
- Replace the MAF sensor
Correct answer: Perform a throttle body idle relearn procedure
Many electronic throttle systems need an idle or throttle relearn after cleaning so the PCM learns the new airflow at closed throttle.
