Question 1

Technician A says an intake manifold vacuum leak may cause a high idle speed. Technician B says a dirty IAC bypass passage will not cause the idle speed to be below specifications. Who is correct?

Accepted Answer

A vacuum leak in the intake manifold can cause unmetered air to enter the engine, resulting in a high idle speed. On the other hand, a dirty or clogged IAC bypass passage can cause the idle speed to be too low or unstable, affecting the engine's ability to idle correctly. Therefore, Technician A is correct and Technician B is wrong.

Question 2

Most heated oxygen sensors have _____ wire(s) connected to them.

Accepted Answer

Most heated oxygen sensors have four wires connected to them: two for the oxygen sensor's operation and two for the heater element.

Question 3

Which of the following would not cause the fuel pressure to drop to zero immediately after engine shut down?

Accepted Answer

A leaking pressure regulator vacuum line would not cause the fuel pressure to drop to zero immediately after engine shutdown. It might affect the fuel pressure regulation when the engine is running, but it wouldn't cause an immediate drop to zero upon shutdown. The other options directly involve components that hold or regulate fuel pressure, and leaks in these components could lead to a loss of pressure.

Question 4

Short-term and long-term fuel trim readings are being discussed. Technician A says a scale of 0 to 255 is used. Technician B says percentage is used. Who is correct?

Accepted Answer

Fuel trim values are typically represented as percentages, not on a scale of 0 to 255. This percentage indicates how much the fuel delivery is being adjusted from the base programmed amount in the vehicle's engine control module (ECM) to maintain an optimal air-fuel ratio.

Question 5

Which of the following should be checked before proceeding with specific fuel injection and electronic control testing?

Accepted Answer

All these components can affect engine performance and should be checked to ensure accurate testing results.

Question 6

Operation of an O₂ sensor is checked with a(n) _____.

Accepted Answer

The O₂ sensor is a voltage sensor, therefore the most appropriate tool to check its operation would be a voltmeter. Both ammeters and ohmmeters measure current and resistance, respectively, which are not the parameters of interest for this sensor. Thus, choice D is not correct, as not all of the answers are applicable.

Question 7

Technician A says the IAC counts should change when the A/C compressor is turned on and off. Technician B says if the scan tester indicates zero IAC counts, there may be an open circuit between the PCM and the IAC motor. Who is correct?

Accepted Answer

Technician A is correct because when the A/C compressor is turned on and off, the load on the engine changes, which affects the idle speed. This change in idle speed is compensated for by the IAC, and thus the IAC counts should change accordingly.

Technician B is also correct because if the scan tester indicates zero IAC counts, there may be an open circuit between the PCM and the IAC motor. This means that the PCM is not able to communicate with the IAC motor, which can affect the engine's idle speed.

Question 8

Technician A says higher than normal idle speed may be caused by an air-conditioning switch that is always closed. Technician B says higher than normal idle speed may be caused by a defective engine coolant temperature sensor. Who is correct?

Accepted Answer

Both technicians are correct. A closed air-conditioning switch can cause the engine control module to increase the idle speed as a compensatory measure. A defective engine coolant temperature sensor can also cause the engine control module to mistakenly increase the idle speed.

Question 9

Which of the following would NOT cause the fuel trim to be out of range?

Accepted Answer

The IAC motor controls the idle speed and has no direct effect on fuel trim. A faulty fuel pressure regulator, restricted fuel filter, or faulty MAF can all cause the engine to receive too much or too little fuel, affecting the fuel trim.

Question 10

Quick tests to a GM hot film mass airflow meter on an engine with idling problems are being performed. Technician A taps on the meter lightly with a plastic mallet. Technician B unplugs the meter. Who is correct?

Accepted Answer

Tapping on the meter can reveal intermittent faults, and unplugging it can help determine if the meter is causing the idling issue by checking if the engine runs better without it.

Question 11

Technician A says a rich air/fuel ratio may be caused by low fuel pump pressure. Technician B says a rich air/fuel ratio may be caused by a defective engine coolant temperature sensor. Who is correct?

Accepted Answer

The answer of Technician A says a rich air/fuel ratio...

Question 12

A technician is testing a fuel injection system with a scan tool. While the engine is running in closed loop, he loosens a vacuum hose to create a small vacuum leak. Initially the STFT rises and the LTFT remains unchanged. After a few minutes the STFT returns to near zero and the LTFT rises. This test proves that ______.

Accepted Answer

The behavior described in the scenario is normal for a fuel injection system. When a vacuum leak is introduced, the Short Term Fuel Trim (STFT) increases to compensate for the additional air entering the engine, indicating the system is running lean temporarily. Over time, the system adjusts, and the Long Term Fuel Trim (LTFT) increases to maintain the correct air-fuel ratio, while the STFT returns to near zero, showing that the system has adapted to the change. This is a normal response of a functioning fuel injection system to a vacuum leak.

Question 13

Technician A says a defective injector may cause cylinder misfiring at idle speed. Technician B says restricted injector tips may result in acceleration stumbles. Who is correct?

Accepted Answer

A defective injector may cause cylinder misfiring at idle speed as suggested by Technician A. Similarly, restricted injector tips may result in acceleration stumbles as pointed out by Technician B. Hence, both technicians are correct.

Question 14

Technician A says the TPREL PID is the PCM's closed throttle reference voltage. Technician B says an idle trim relearn procedure should be done whenever a part of the IAC system is replaced or a repair is made to something that affects idle speed. Who is correct?

Accepted Answer

Technician A is correct because the TPREL PID does indeed refer to the PCM's closed throttle reference voltage. Technician B is also correct because an idle trim relearn procedure should be conducted after any repair or replacement of a part of the IAC system that might affect idle speed. Therefore, both technicians are correct.

Question 15

Technician A says if a lean condition caused by a vacuum leak, restricted injectors, dirty MAF, clogged fuel filter, or bad fuel pump exists, the LTFT will have a negative number. Technician B says when the O2 sensor has a rich signal, the STFT goes lean to adjust the air/fuel ratio. Who is correct?

Accepted Answer

The answer of Technician A says if a lean condition...

Question 16

Technician A says throttle body cleaner may be used to clean the IAC motor internal components. Technician B says on some vehicles, IAC motor damage can occur if the pintle is cleaned with throttle body cleaner. Who is correct?

Accepted Answer

Throttle body cleaner should not be used to clean the pintle of an IAC motor as it can cause damage to the component. Therefore, Technician B is correct while Technician A is wrong.

Question 17

The voltage signal from most O₂ sensors varies between _____ and _____ volt(s).

Accepted Answer

The answer of The voltage signal from most O₂ sensors...

Question 18

The activity of an O2 sensor is best monitored with a ______.

Accepted Answer

The answer of The activity of an O2 sensor is...

Question 19

Idle learn procedures for ETC [throttle-by-wire]-equipped vehicles are being discussed. Technician A says the learn or relearn procedure should be completed anytime the PCM has been replaced or updated, or after the throttle body has been cleaned or replaced. Technician B says the system does not need to relearn if the battery has been disconnected. The system will automatically go through the procedure when the engine is restarted.Who is correct?

Accepted Answer

The answer of Idle learn procedures for ETC [throttle-by-wire]-equipped vehicles...

Question 20

A rich air/fuel mixture is being discussed. Technician A says oxygen content in the exhaust will be high. Technician B says the O₂ sensor voltage will be high. Who is correct?

Accepted Answer

Technician B is correct because a rich air/fuel mixture will result in a high voltage signal from the oxygen sensor (often referred to as O2 sensor or lambda sensor). This is because the sensor detects a low level of oxygen in the exhaust due to the excess fuel, leading to a higher voltage signal indicating a rich condition. Technician A is incorrect because a rich mixture will have low, not high, oxygen content in the exhaust.

Quiz 31: Fuel Injection System Diagnosis and Service

Technician A says an intake manifold vacuum leak may cause a high idle speed. Technician B says a dirty IAC bypass passage will not cause the idle speed to be below specifications. Who is correct?

Most heated oxygen sensors have _____ wire(s) connected to them.

Which of the following would not cause the fuel pressure to drop to zero immediately after engine shut down?

Short-term and long-term fuel trim readings are being discussed. Technician A says a scale of 0 to 255 is used. Technician B says percentage is used. Who is correct?

Which of the following should be checked before proceeding with specific fuel injection and electronic control testing?

Operation of an O₂ sensor is checked with a(n) _____.

Technician A says the IAC counts should change when the A/C compressor is turned on and off. Technician B says if the scan tester indicates zero IAC counts, there may be an open circuit between the PCM and the IAC motor. Who is correct?

Technician A says higher than normal idle speed may be caused by an air-conditioning switch that is always closed. Technician B says higher than normal idle speed may be caused by a defective engine coolant temperature sensor. Who is correct?

Which of the following would NOT cause the fuel trim to be out of range?

Quick tests to a GM hot film mass airflow meter on an engine with idling problems are being performed. Technician A taps on the meter lightly with a plastic mallet. Technician B unplugs the meter. Who is correct?

Technician A says a rich air/fuel ratio may be caused by low fuel pump pressure. Technician B says a rich air/fuel ratio may be caused by a defective engine coolant temperature sensor. Who is correct?

Technician A says a defective injector may cause cylinder misfiring at idle speed. Technician B says restricted injector tips may result in acceleration stumbles. Who is correct?

Technician A says the TPREL PID is the PCM's closed throttle reference voltage. Technician B says an idle trim relearn procedure should be done whenever a part of the IAC system is replaced or a repair is made to something that affects idle speed. Who is correct?

Technician A says if a lean condition caused by a vacuum leak, restricted injectors, dirty MAF, clogged fuel filter, or bad fuel pump exists, the LTFT will have a negative number. Technician B says when the O2 sensor has a rich signal, the STFT goes lean to adjust the air/fuel ratio. Who is correct?

Technician A says throttle body cleaner may be used to clean the IAC motor internal components. Technician B says on some vehicles, IAC motor damage can occur if the pintle is cleaned with throttle body cleaner. Who is correct?

The voltage signal from most O₂ sensors varies between _ and _ volt(s) .

The activity of an O2 sensor is best monitored with a ______.

A rich air/fuel mixture is being discussed. Technician A says oxygen content in the exhaust will be high. Technician B says the O₂ sensor voltage will be high. Who is correct?

Quiz 31: Fuel Injection System Diagnosis and Service

Technician A says an intake manifold vacuum leak may cause a high idle speed. Technician B says a dirty IAC bypass passage will not cause the idle speed to be below specifications. Who is correct?

Most heated oxygen sensors have _____ wire(s) connected to them.

Which of the following would not cause the fuel pressure to drop to zero immediately after engine shut down?

Short-term and long-term fuel trim readings are being discussed. Technician A says a scale of 0 to 255 is used. Technician B says percentage is used. Who is correct?

Which of the following should be checked before proceeding with specific fuel injection and electronic control testing?

Operation of an O2 sensor is checked with a(n) _____.

Technician A says the IAC counts should change when the A/C compressor is turned on and off. Technician B says if the scan tester indicates zero IAC counts, there may be an open circuit between the PCM and the IAC motor. Who is correct?

Technician A says higher than normal idle speed may be caused by an air-conditioning switch that is always closed. Technician B says higher than normal idle speed may be caused by a defective engine coolant temperature sensor. Who is correct?

Which of the following would NOT cause the fuel trim to be out of range?

Quick tests to a GM hot film mass airflow meter on an engine with idling problems are being performed. Technician A taps on the meter lightly with a plastic mallet. Technician B unplugs the meter. Who is correct?

Technician A says a rich air/fuel ratio may be caused by low fuel pump pressure. Technician B says a rich air/fuel ratio may be caused by a defective engine coolant temperature sensor. Who is correct?

Technician A says a defective injector may cause cylinder misfiring at idle speed. Technician B says restricted injector tips may result in acceleration stumbles. Who is correct?

Technician A says the TPREL PID is the PCM's closed throttle reference voltage. Technician B says an idle trim relearn procedure should be done whenever a part of the IAC system is replaced or a repair is made to something that affects idle speed. Who is correct?

Technician A says throttle body cleaner may be used to clean the IAC motor internal components. Technician B says on some vehicles, IAC motor damage can occur if the pintle is cleaned with throttle body cleaner. Who is correct?

The voltage signal from most O2 sensors varies between _____ and _____ volt(s) .

The activity of an O2 sensor is best monitored with a ______.

A rich air/fuel mixture is being discussed. Technician A says oxygen content in the exhaust will be high. Technician B says the O2 sensor voltage will be high. Who is correct?

Operation of an O₂ sensor is checked with a(n) _____.

The voltage signal from most O₂ sensors varies between _ and _ volt(s) .

A rich air/fuel mixture is being discussed. Technician A says oxygen content in the exhaust will be high. Technician B says the O₂ sensor voltage will be high. Who is correct?