Engine Performance Flashcards

Fuel pressure is in spec at idle but drops well below spec under hard acceleration. What is the most likely cause?

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.

After shutdown, fuel pressure bleeds off quickly and the engine is hard to start when hot. Which cause is most likely?

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.

An injector balance test shows one injector with a much smaller pressure drop than the others. What does this indicate?

Answer: The injector is restricted or clogged

A smaller pressure drop means less fuel passed through that injector, which points to a restriction.

A contaminated MAF sensor typically causes what fuel trim pattern?

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.

Which condition causes the PCM to run in open loop?

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.

A variable valve timing (VVT) code appears after an oil change with the wrong oil viscosity. Why?

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.

A drive-by-wire throttle body is cleaned and the engine now idles erratically. What step is often required?

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.