ASE A8 Engine Performance Practice Test

11. A scan indicates that an engine's air-fuel ratio is rich. Which of the following would result in this condition?

  • A. A faulty (ECT) Engine Coolant Temperature Sensor.
  • B. A faulty thermostat.
  • C. Both A and B.
  • D. Neither A or B.


Answer A is wrong. The ECM uses the signal from the ECT sensor to indicate engine temperature. It enrichens the air-fuel ratio at start-up and when the engine is cold.

Answer B is wrong. A stuck open thermostat results in a rich air-fuel mixture because the engine would not reach normal operating temperature.

Answer C is correct. Both A and B are correct.

Answer D is wrong. A (14.7:1) air-fuel ratio is considered the perfect mixture or a stoichiometric air-fuel ratio.

12. An engine has reached normal operating temperature. A graph of a four-wire oxygen sensor signal cycles between .150 Volt and .850 Volt. Which of the following is correct?

A. This oxygen sensor contains a heater circuit.

B. The oxygen sensor has failed and is biased/stuck rich.

  • A. Answer A
  • B. Answer B
  • C. Both A and B
  • D. Neither A or B


Answer A is correct. A four-wire oxygen sensor contains a heater circuit to quickly bring it to operating temperature so the fuel delivery system can enter closed-loop mode.

Answer B is wrong. Oxygen passing through the element affects the voltage signal to the ECM. The signal cycles and fluctuates between rich and lean.

Answer C is wrong. Oxygen sensors produce a high voltage signal (.90) when they sense low oxygen and a low voltage signal (.10) when they sense high oxygen, and the engine is running lean.

Answer D is wrong. While the engine is cold, at wide-open throttle, or under high load conditions, the ECM is in the open-loop mode of operation. When the oxygen sensors are above 600° F, the system enters closed-loop.

Diagram of automotive PCM engine control system.

13. The component labeled A in the schematic diagram above is:

  • A. A member of the primary ignition system.
  • B. A member of the secondary ignition system.
  • C. Switched by the secondary circuit.
  • D. Both A and B.


Answer A is wrong. The ignition coil is part of the primary and secondary circuits. The primary circuit is switched on and off by the ignition control module using signals from the crankshaft and camshaft position sensors.

Answer B is wrong. Today's coils multiply 12 volts battery voltage to 25k to 40k volts through induction.

Answer C is wrong. An ignition coil is controlled or switched by the primary circuit.

Answer D is correct. Both A and B are correct. See the read more section to learn more about ignition coils.

14. A vehicle emission test indicates high levels of (NOx) nitrogen oxides. Which of the following is MOST likely causing this condition?

  • A. A leaking fuel injector.
  • B. A bad ignition coil.
  • C. A random misfire.
  • D. A stuck closed EGR valve.


Answer A is wrong. A leaking fuel injector results in high hydrocarbon emissions, a rich air-fuel ratio, and black sooty exhaust.

Answer B is wrong. Without ignition, raw unburned fuel enters the exhaust.

Answer C is wrong. A misfire at one or more cylinders results in excessive HC's due to unburned fuel in the exhaust.

Answer D is correct. A stuck closed EGR valve results in high cylinder temperatures. This excessive heat creates high levels of (NOx) in the vehicle's emissions.

15. A technician is using a digital multimeter to perform a voltage drop test on a ground circuit. Technician A says to set the meter for measuring ohms, and voltage must be present in the circuit. Technician B says a voltage drop test tests for excessive resistance in a circuit. Who is correct?

  • A. Technician A
  • B. Technician B
  • C. Both A and B
  • D. Neither A or B


Answer A is wrong. A voltage drop test would verify a weak or corroded ground circuit or connection.

Answer B is correct. A voltage drop test is a great way to find resistance in a circuit. Never use an ohmmeter when voltage is present; doing so can damage the meter.

Answer C is wrong. Voltage drop is the quantity of voltage dropped or lost as it passes through a component or resistance in a circuit. Voltage must be present in the circuit to perform a voltage drop test.

Answer D is wrong. Perform a voltage drop test to find excessive resistance in a circuit.