ASE A8 Engine Performance Practice Test

Diagram of automotive PCM engine control system.

6. There is excessive resistance found at point A in the diagram above. All of the following statements are true EXCEPT.

  • A. This will result in a lean air/fuel ratio.
  • B. This will result in delayed torque converter engagement.
  • C. This will result in delayed engine cooling fan operation.
  • D. This is the ground connection for an (NTC) negative coefficient temperature engine coolant temperature sensor.

6.

Answer A is correct. Faulty inputs result in incorrect actuator adjustments. This is true for fuel injector pulse width and negative or positive fuel trim.

Answer B is wrong. An engine coolant temperature sensor is an NTC or negative temperature coefficient sensor. These sensors have less resistance as temperature increases. If there is excessive resistance in this circuit the PCM will think the engine is cooler than it actually is and delay torque converter engagement.

Answer C is wrong. This will result in delayed engine cooling fan engagement.

Answer D is wrong. Answer D is wrong, A is correct. This is because this is an EXCEPT type question. Look for the answer that doesn't fit. This is the ground connection for an (NTC) negative coefficient temperature (ECT) engine coolant temperature sensor.

7. The PCM uses all of these inputs to calculate air fuel ratio and speed control EXCEPT:

  • A. An engine coolant temperature sensor. (ECT)
  • B. An exhaust gas recirculation valve. (EGR)
  • C. An intake air temperature sensor. (IAT)
  • D. A throttle position sensor. (TPS)

7.

Answer A is wrong. The (ECT) Engine Coolant Temperature Sensor input is used to adjust air fuel ratio.

Answer B is correct. The exhaust gas recirculating (EGR) valve is not a sensor, it's a valve that recirculates exhaust into the intake manifold. The PCM is using fuel mixture sensors to calculate injector pulse, engine idle speed and air fuel ratio. The EGR valve is tasked with keeping the cylinder's cool preventing harmful NOx gases from forming.

Answer C is wrong. The (IAT) Intake Air Temperature Sensor input is used to adjust air fuel ratio.

Answer D is wrong. The (TPS) Throttle Posistion Sensor is used to adjust air fuel ratio.

8. Two technicians are discussing a turbocharged engine. Technician A says the wastegate is a component used to control boost pressure. Technician B says when opened, the wastegate diverts exhaust gas from the turbine wheel. Who is correct?

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

8.

Answer A is wrong. The wastegate is controlled by a valve and releases excess pressure back to the exhaust stream. This device along with it's control valve are used to control intake manifold pressure by diverting pressure away from the turbine wheel and back into the exhaust stream.

Answer B is wrong. They are both right. When the gate opens, exhaust by-passes the turbine wheel preventing an increase in intake manifold boost pressure.

Answer C is correct. Both technicians are correct.

Answer D is wrong. Both technicians are correct.

9. Technician A says when the vacuum hose is disconnected from a vacuum operated fuel pressure regulator, fuel pressure should increase. Technician B says a restricted fuel return line will cause low fuel rail pressure. Who is correct?

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

9.

Answer A is correct. As intake manifold vacuum changes so does fuel rail's pressure. This is because as the throttle plate opens, vacuum drops and the fuel pressure regulator's diaphragm blocks return flow. This causes fuel pressure to rise and compensate for the increased demand.

Answer B is wrong. A restricted fuel return line may result in a rich, not lean fuel condition. The fuel return line needs to flow freely for gas to flow back into the tank. A kink alters the pressure and will cause higher pressure at the upstream end of the restriction and lower pressure at the downstream end.

Answer C is wrong. Technician A is correct.

Answer D is wrong. Technician A is correct.

10. An EFI engine with a Mass Air Flow sensor is scanned and the LTFT is found to be 24% at idle. This percentage decreases as the engine's speed is increased to 1500 rpm. All of the following will result in this fuel trim percentage EXCEPT:

  • A. A faulty oxygen sensor.
  • B. A dripping fuel injector.
  • C. A dirty Mass Air Flow sensor.
  • D. A leaking vacuum line.

10.

Answer A is wrong. This indicates a lean fuel condition. The PCM is adding fuel (positive + fuel trim) to compensate for a lean fuel condition.

Answer B is correct. A leaking or dripping fuel injector will result in a rich air fuel ratio. The PCM will reduce the amount of fuel (negative - fuel trim) by decreasing injector pulse width. This leaking injector would result in negative LTFT.

Answer C is wrong. A dirty MAF sensor will result in positive LTFT. Cleaning a hot wire MAF sensor can correct and restore the sensor's signal to the PCM.

Answer D is wrong. A vacuum leak will result in excessive positive fuel trim. LTFT is a constant and stays in memory after the engine has been shut down. It's obtained from oxygen sensor input while the system is in closed loop.