ASE A8 Engine Performance Practice Test
21. An ohmmeter indicates excessive resistance at point A in the schematic diagram above. All of the following statements are true about this circuit EXCEPT:
- A. Point A is the ground connection for an (NTC) negative temperature coefficient sensor.
- B. Too much resistance at point A will result in delayed torque converter lockup-clutch engagement.
- C. Too much resistance at point A will affect the air-fuel ratio.
- D. Too much resistance at point A will result in continuous engine cooling fan operation.
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.
22. 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 and 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 senses the temperature of the engine's coolant, lowering its resistance as the engine warms to operating temperature.
23. 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. An underboost condition usually occurs when the wastegate stays open or opens too early. A sticking wastegate or wastegate linkage will also result in an underboost.
Answer B is correct. A faulty valve, a disconnected vacuum line, or a leaking wastegate diaphragm will cause an overboost condition. If the wastegate does not open or only partially opens, an overboost condition will occur. Overboost can damage the turbocharger and the engine.
Answer C is wrong. Use a vacuum gauge to check intake manifold pressure. Check for disconnected vacuum lines or a leaking manifold gasket.
Answer D is wrong. Premature turbocharger failure is typically due to a lack of maintenance or contaminated and unchanged oil, resulting in bearing damage.
24. 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 kinked and restricted fuel return line raises pressure at the fuel rail.
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's fuel inside the line.
Answer C is correct. A clogged or faulty fuel injector results in a misfire at that cylinder.
Answer D is wrong. With a fuel pressure tester attached to the service port on the rail, disconnect the vacuum hose from the regulator. There should be an increase in fuel pressure. Reconnecting the hose should return fuel pressure to specifications.
25. A scanner is connected to a vehicle's data link connector to monitor fuel trim values. The display indicates a long term fuel trim of 24% at idle. This percentage decreases when the engine speed is increased to 1500 RPM. Which of the following could be causing this condition?
- A. High fuel pressure.
- B. A disconnected vacuum hose.
- C. A ruptured fuel pressure regulator diaphragm.
- 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 ECM is subtracting fuel; for example, -24%.
Answer B is correct. A vacuum leak results in excessive positive fuel trim. A positive number indicates the ECM is adding fuel to compensate for a lean fuel condition
Answer C is wrong. A ruptured fuel pressure regulator diaphragm causes high fuel pressure and a rich air-fuel ratio.
Answer D is wrong. A leaking fuel injector produces a rich air-fuel ratio and a negative LTFT.