ASE A8 Engine Performance Practice Test

21. An engine's control module uses the speed-density method to estimate airflow into the engine. Technician A says this engine relies on the MAP and IAT sensors to adjust the air-fuel ratio. Technician B says the MAP sensor indicates the intake manifold pressure to the ECM. Who is correct?

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


Answer A is wrong. The speed-density method relies primarily on (IAT) intake air temperature, (MAP) manifold pressure, and (CKP) engine speed to make calculations.

Answer B is wrong. A (MAP) Manifold Absolute Pressure sensor sends a voltage signal to the ECM regarding intake manifold vacuum/pressure.

Answer C is correct. Both technicians are correct.

Answer D is wrong. Newer engines typically have (MAF) Mass Air Flow sensors that take measurements directly from the air tube.

22. There is a misfire at the #3 cylinder of a COP ignition system. A voltage test indicates no power at the Bat+ terminal of the coil's plug. Which of these could be the cause?

  • A. Resistance in the primary circuit.
  • B. Resistance in the secondary circuit.
  • C. A leak in the secondary insulation.
  • D. An open between the coil and its 12-volt source.


Answer A is wrong. Resistance causes a weak spark at that cylinder.

Answer B is wrong. Resistance in the secondary or boot will result in spark leaking at that cylinder.

Answer C is wrong. A COP ignition system eliminates the distributor and the secondary ignition wires.

Answer D is correct. An open circuit between the coil and the ECM is the most likely cause of the no-spark condition. An open in the wire or connector between the power source, ground, or ECM and the coil would result in no-spark at that cylinder.

23. The OBD II Enhanced Evaporative Emissions monitor has illuminated the check engine light and set fault code P0457 indicating a gross leak has been detected. Which of the following is MOST likely causing this leak?

  • A. A crimp in the vent hose.
  • B. A saturated canister.
  • C. A stuck closed purge valve.
  • D. A loose fuel filler cap.


Answer A is wrong. The vent valve is a normally open valve. Blockage like a kink in the vent hose would restrict fresh air flow into the canister.

Answer B is wrong. A saturated canister will not result in a gross leak detected code.

Answer C is wrong. The purge valve is a normally closed valve that vents fuel vapor into the intake manifold when activated.

Answer D is correct. A loose or faulty gas cap will result in a gross-leak-detected code. In addition, some vehicles illuminate a unique loose gas cap light on the instrument panel when the cap is left loose.

24. A vehicle with a four-cylinder EFI engine has a clogged catalytic converter. Scan tool data indicates the engine has a Long Term Fuel Trim (LTFT) of -19 % at idle. Technician A says a LTFT of -19 % means the ECM is adding fuel to the air-fuel mixture. Technician B says a rich air-fuel ratio has caused the catalytic converter to overheat. Who is correct?

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


Answer A is wrong. When a - minus precedes the percentage, the ECM is subtracting fuel from the injector pulse width. When the O2 sensor indicates low oxygen content in the exhaust stream, the ECM will compensate for this rich signal by leaning the mixture to return STFT to an acceptable range.

Answer B is correct. A rich air-fuel mixture will result in excess unburned fuel (HC's) entering the catalytic converter causing it to overheat.

Answer C is wrong. Technician B is correct.

Answer D is wrong. Technician B is correct.

25. A vehicle has a fuel trim of -5 % at idle and -25 % at cruise. Scan tool data indicates a steady oxygen sensor output voltage of 800Mv. When a vacuum hose is disconnected from the intake manifold, there is no change in the oxygen sensor's voltage signal. This condition is caused by:

  • A. An intake manifold leak.
  • B. A faulty throttle position sensor.
  • C. A faulty oxygen sensor.
  • D. A kink in the fuel return line.


Answer A is wrong. Short-term fuel trim is temporary changes of fuel pulse width to compensate for a change in speed or load like climbing a hill.

Answer B is wrong. Long-term fuel trim is more permanent. The ECM will adjust the fuel trim to compensate for continuous operational trends like climate or high altitudes.

Answer C is correct. Disconnecting a vacuum line from the intake manifold should cause the O2 sensor signal to change.

Answer D is wrong. A restricted fuel return line results in a rich air-fuel mixture, but removing the vacuum hose should affect the oxygen sensor's output. Read more about fuel trim in the Read more... section.