NAPA Engine Performance 2 — Questions and Answers
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?
- A vacuum leak (Correct answer)
- A restricted fuel filter
- A weak fuel pump
- A clogged catalytic converter
Correct 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.
Question 2: A zirconia upstream O2 sensor that stays at about 0.1-0.2 volts indicates what condition?
- Rich mixture
- Lean mixture (Correct answer)
- Normal closed-loop switching
- Sensor heater circuit open
Correct answer: Lean mixture
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?
- Burned exhaust valve
- Restricted exhaust
- Slightly rich or lean mixture (carburetion/fuel issue) (Correct answer)
- Late ignition timing
Correct 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.
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?
- Worn piston rings
- Valve or head gasket leak (Correct answer)
- Worn camshaft lobe on the intake
- Faulty spark plug
Correct 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.
Question 5: Which DTC is set when the PCM detects a random/multiple cylinder misfire?
- P0300 (Correct answer)
- P0171
- P0420
- P0401
Correct answer: P0300
P0300 is the generic code for random or multiple cylinder misfire detected.
Question 6: Which sensor does a speed-density fuel injection system mainly use to calculate engine load?
- Mass airflow sensor
- Manifold absolute pressure sensor (Correct answer)
- Throttle position sensor
- Engine coolant temperature sensor
Correct answer: Manifold absolute pressure sensor
Speed-density systems calculate air mass from MAP, rpm, and intake air temperature.
Question 7: With the key on and engine off, a MAP sensor should read close to what value?
- Zero kPa
- Barometric pressure (Correct answer)
- Manifold vacuum at idle
- Boost pressure
Correct answer: Barometric pressure
With the engine stopped, manifold pressure equals atmospheric pressure, so the MAP reads barometric pressure.
