ASE A8 Engine Performance Practice Test
6. An engine has a rich air-fuel ratio. There is excessive resistance found at point A in the schematic diagram above. All of the following statements regarding this circuit are true EXCEPT:
- A. Point A is the ground connection for the (NTC) negative temperature coefficient sensor.
- B. Too much resistance at point A will result in delayed torque converter engagement.
- C. Too much resistance at point A will result in delayed engine cooling fan operation.
- D. The ECM provides a 12-volt reference signal.
Answer A is wrong. Point A is the ground connection for the Engine Coolant Temperature Sensor, an (NTC) negative temperature coefficient sensor. The engine runs a startup strategy that enriches the air-fuel ratio when the engine's coolant is cold. These sensors have less resistance as temperature increases.
Answer B is wrong. The ECT sensor affects several different systems, like air conditioning and transmission. This signal affects EGR flow, enrich fuel mixtures, and delay torque converter, or A/C compressor engagement.
Answer C is wrong. The ECM uses different strategies programmed into its lookup tables for hot and cold operating conditions. This resistance results in delayed engine cooling fan engagement.
Answer D is correct. This is an EXCEPT-type question. The (ECT) engine coolant temperature sensor is a variable resistor that uses resistance to change a 5-volt reference signal from the ECM.
7. An engine is misfiring and stumbling at idle. Which of the following is MOST likely causing this condition?
- A. A faulty (TPS) throttle position sensor.
- B. A faulty (IAT) intake air temperature sensor.
- C. A faulty (EGR) exhaust gas recirculation valve.
- D. A faulty (ECT) engine coolant temperature sensor.
Answer A is wrong. The (TPI) throttle position sensor is a potentiometer that indicates the throttle plate's angle to the PCM. The ECM uses this sensor's information along with several others to control injector pulse width and timing.
Answer B is wrong. The (IAT) or intake air temperature sensor is a thermistor that changes its signal according to intake air temperature. The PCM provides a richer air-fuel ratio as cool air passes the sensor. Cold air is denser and contains more oxygen than warm air does.
Answer C is correct. The exhaust gas recirculation valve regulates a small portion of exhaust into the combustion chamber. A stuck open EGR valve results in a rough idle. A stuck closed EGR valve results in high cylinder temperatures, NOx production, and engine knock.
Answer D is wrong. The (ECT) Engine Coolant Temperature Sensor continually sensing the temperature of the engine's coolant lowering its resistance as the engine warms to operating temperature.
8. The turbocharger's wastegate diaphragm is leaking. This leak will result in:
- A. An underboost condition.
- B. An overboost condition.
- C. Low intake manifold pressure.
- D. Oil consumption.
Answer A is wrong. The wastegates actuator and actuator diaphragm control a turbocharger's output. A stuck open wastegate results in underboost, poor engine performance, and a lack of horsepower.
Answer B is correct. The actuator opens the wastegate to divert exhaust away from the turbine, depriving it of its source of energy. A leaking or ruptured wastegate diaphragm doesn't divert enough exhaust resulting in an overboost condition.
Answer C is wrong. This compressed charge is denser in comparison to a conventional engine that relies on the downward stroke of the piston. The denser charge is forced into the cylinders by increased intake manifold pressure to provide additional engine horsepower.
Answer D is wrong. Most turbocharger failures are oil related. Contaminated and unchanged oil eventually results in bearing failure. Turbocharger seals are affected, resulting in oil consumption and blue-gray exhaust. Oil is mixing with the air-fuel charge and burning off in the cylinder.
9. An engine with (SFI) Sequential Fuel Injection has poor fuel mileage. A fuel pressure test indicates high fuel pressure at the fuel rail. Which of the following is LEAST likely to result in this condition?
- A. A kink in the fuel return line.
- B. A ruptured fuel regulator diaphragm.
- C. A clogged fuel injector.
- D. A disconnected fuel regulator vacuum hose.
Answer A is wrong. A restricted fuel return line may result in a rich fuel condition. The fuel return line needs to flow freely for fuel to flow back into the tank.
Answer B is wrong. A ruptured fuel pressure regulator diaphragm results in high fuel pressure; remove the regulator's vacuum hose to see if there if there's fuel inside the hose.
Answer C is correct. A clogged or faulty fuel injector results in a misfire at that cylinder.
Answer D is wrong. Today's engines are port fuel injected, and the fuel injector's tips extend into the intake manifold just above the intake valve. A vacuum controlled fuel pressure regulator is used to compensate for the fluctuations in vacuum pressure.
10. An engine with sequential fuel injection is scanned, and the LTFT is 24% at idle. This percentage decreases when the engine speed is increased to 1500 RPM. Which of the following will result in this condition?
- A. High fuel pressure.
- B. A disconnected vacuum hose.
- C. A ruptured fuel pressure regulator diapragm.
- D. A dripping fuel injector.
Answer A is wrong. High fuel pressure results in a rich fuel condition. The percentage will have a minus sign (-) preceding the number because the PCM is subtracting fuel; for example, -24%.
Answer B is correct. A vacuum leak results in excessive positive fuel trim. A positive number like (24%) indicates the PCM is adding fuel to compensate for a lean fuel condition. 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.
Answer C is wrong. A ruptured fuel pressure regulator diapragm results in high fuel pressure, not a lean fuel ratio.
Answer D is wrong. A leaking or dripping fuel injector results in a rich air-fuel ratio. The PCM reduces the amount of fuel (negative - fuel trim) by decreasing the injector pulse width. This leaking injector would result in a negative LTFT.