NAPA Engine Performance 2

Question 1

A scan tool shows long-term fuel trim at +18% on both banks at idle, dropping to +3% at 2,500 rpm. What is the most likely cause?

Accepted Answer

A vacuum leak

Answer

Unmetered air from a vacuum leak has the most effect at idle and becomes a small share of total airflow at higher rpm.

Question 2

A zirconia upstream O2 sensor that stays at about 0.1-0.2 volts indicates what condition?

Accepted Answer

Lean mixture

Answer

Low zirconia sensor voltage means excess oxygen in the exhaust, which is a lean condition.

Question 3

On a healthy warm engine at idle, a vacuum gauge needle that floats slowly between 14 and 16 in. Hg most likely indicates what?

Accepted Answer

Slightly rich or lean mixture (carburetion/fuel issue)

Answer

A slow drift of a few inches usually points to an air/fuel mixture problem rather than a mechanical fault.

Question 4

A cylinder shows 40 psi on a dry compression test and 45 psi on a wet test, while the other cylinders read 165 psi. What is the most likely cause?

Accepted Answer

Valve or head gasket leak

Answer

Little gain on the wet test means the rings are not the leak path, so the problem is valves or the head gasket.

Question 5

Which DTC is set when the PCM detects a random/multiple cylinder misfire?

Accepted Answer

P0300

Answer

P0300 is the generic code for random or multiple cylinder misfire detected.

NAPA - National Automotive Parts Association Certified Mechanic Practice Test