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 ECT sensor is an NTC or negative temperature coefficient thermistor located in the coolant stream on or around the thermostat housing. If the ECT were faulty and the PCM were receiving a signal indicating a colder than actual engine, the mixture would be enriched.

Answer B is wrong. A stuck closed thermostat would result in an overheated engine. A stuck open thermostat results in a richer than normal fuel mixture because the engine would not reach normal operating temperature.

Answer C is correct. A (14.7:1) air-furl ratio is considered the perfect blend of air to fuel mixture or stoichiometric air-fuel ratio.

Answer D is wrong. Both A and B are correct.

12. Two technicians are discussing a four wire oxygen sensor. Technician A says this oxygen sensor contains a heater circuit. Technician B says the engine's control module uses the oxygen sensor's voltage signal to adjust the air fuel ratio in closed loop. Who is correct?

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


Answer A is wrong. Both technicians are correct. The oxygen sensor contains a heater to bring it to operating temperature sooner.

Answer B is wrong. Both technicians are correct.

Answer C is correct. An oxygen sensor's heater circuit is designed to bring the system to closed loop quickly. It sends a voltage signal to the engine control module in relation to the oxygen content of the exhaust stream.

Answer D is wrong. Both technicians are correct.

Diagram of automotive PCM engine control system.

13. Component A in the diagram above is:

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


Answer A is wrong. Both A and C are correct. An ignition coil is part of the primary and the secondary circuits.

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

Answer C is wrong. It is part of both circuits.

Answer D is correct. Both Technicians A and C are correct.

14. An emissions test is performed on a six cylinder engine. The test indicates high levels of NOx. The MOST likely cause of this condition is:

  • A. A rich air fuel mixture.
  • B. A clogged catalytic converter.
  • C. A cylinder misfire.
  • D. A stuck closed EGR valve.


Answer A is wrong. A rich fuel mixture will increase HC or hydrocarbon emissions.

Answer B is wrong. A clogged catalytic converter will result in a loss of power.

Answer C is wrong. A cylinder misfire will result in excessive HC's due to the increase of unburned fuel.

Answer D is correct. A stuck closed EGR valve will result in high cylinder temperatures. This excessive heat results in high levels of NOx in the emissions.

15. A technician is using a digital multimeter to test a ground circuit for excessive resistance. Technician A says to perform a voltage drop test on this circuit. Technician B says an ohmmeter is used and voltage must be present while performing this resistance test. Who is correct?

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


Answer A is correct. A voltage drop test would verify a poor or corroded ground circuit or connection.

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

Answer C is wrong. Technician A is correct.

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