ASE A8 Engine Performance Exam

ASE A8 Engine Performance Exam Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary function of the oxygen sensor in an engine

management system?

• Measure engine coolant temperature
• Measure manifold absolute pressure
• Measure throttle position
• Measure exhaust oxygen content

Rationale: The oxygen sensor monitors the amount of oxygen in the

exhaust, allowing the ECM to adjust the air/fuel mixture for optimal combustion and emissions.

• A customer complains of poor fuel economy. Which of the following is

the most likely cause?

• Faulty crankshaft sensor
• Dirty fuel injectors 1 / 4

• Worn timing chain
• Broken EGR valve

Rationale: Dirty or clogged fuel injectors can cause poor atomization of

fuel, leading to incomplete combustion and decreased fuel efficiency.

• What does the MAP sensor measure?
• Engine speed
• Manifold absolute pressure
• Air temperature
• Oxygen content in exhaust

Rationale: The MAP (Manifold Absolute Pressure) sensor measures the

pressure inside the intake manifold, helping the ECM calculate engine load for proper fuel delivery and ignition timing.

• Which component is primarily responsible for controlling idle speed

electronically?

• Throttle body
• Fuel pump
• Idle Air Control (IAC) valve
• Camshaft position sensor

Rationale: The IAC valve regulates the amount of air bypassing the throttle

plate to maintain a stable engine idle under various conditions.

5. A misfire at high RPM with no trouble codes is most likely caused by:

• Faulty crankshaft position sensor
• Weak ignition coil 2 / 4

• Clogged EGR valve
• Low oil pressure

Rationale: High-RPM misfires with no codes often indicate an ignition

problem such as a weak coil or failing spark plug wires.

• What is the typical voltage range of a Hall-effect crankshaft position

sensor?

A. 0–5 V

B. 0–12 V

C. 12–24 V

D. 5–24 V

Rationale: Hall-effect sensors produce a square-wave signal that switches

between 0 and 12 volts, which the ECM uses to determine crankshaft position.

• Which condition is most likely if an engine has a lean air/fuel mixture?
• Black smoke from exhaust
• Rich fuel smell
• Misfire under load
• Hard starting in cold weather

Rationale: A lean mixture means insufficient fuel relative to air, often

causing misfires and poor acceleration under load.

• What is the purpose of a knock sensor?
• Control fuel pressure
• Measure exhaust temperature 3 / 4

• Monitor coolant temperature
• Detect engine detonation

Rationale: The knock sensor detects engine pinging or detonation,

allowing the ECM to retard ignition timing to prevent engine damage.

• Which of the following could cause excessive NOx emissions?
• Faulty oxygen sensor
• Clogged catalytic converter
• Lean air/fuel mixture
• Low fuel pressure

Rationale: A lean mixture burns hotter, which increases the formation of

nitrogen oxides (NOx) in the exhaust.

• What is a common symptom of a failing mass airflow (MAF) sensor?
• Hard starting
• Hesitation or stalling
• Engine overheating
• Excessive oil consumption

Rationale: A faulty MAF sensor can send incorrect airflow data to the ECM,

causing hesitation, stalling, or poor performance.

• What effect does a vacuum leak have on engine operation?
• Rich mixture and black smoke
• Lean mixture and high idle
• Hard starting in cold weather
• Misfire only at low RPM
• / 4