Engine Performance Flashcards

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?

Answer: A vacuum leak

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

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

Answer: Lean mixture

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

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

Answer: Slightly rich or lean mixture (carburetion/fuel issue)

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

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?

Answer: Valve or head gasket leak

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

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

Answer: P0300

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

Which sensor does a speed-density fuel injection system mainly use to calculate engine load?

Answer: Manifold absolute pressure sensor

Speed-density systems calculate air mass from MAP, rpm, and intake air temperature.

With the key on and engine off, a MAP sensor should read close to what value?

Answer: Barometric pressure

With the engine stopped, manifold pressure equals atmospheric pressure, so the MAP reads barometric pressure.