NAPA Engine Performance 5

Question 1

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

Accepted Answer

Restricted fuel filter or weak pump

Answer

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?

Accepted Answer

Leaking injector or faulty check valve

Answer

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?

Accepted Answer

The injector is restricted or clogged

Answer

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?

Accepted Answer

Positive (lean) trims, especially at higher airflow

Answer

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?

Accepted Answer

Cold engine before O2 sensors reach operating temperature

Answer

The PCM stays in open loop until the O2 sensors and engine are warm enough to give reliable feedback.

NAPA - National Automotive Parts Association Certified Mechanic Practice Test