ASE4 Fuel, Air Induction, and Exhaust Systems 2 — Questions and Answers
Question 1: A MAF sensor reads 2 g/s at idle but the normal range is 3–7 g/s. The engine runs lean. This indicates the MAF sensor is:
- Reading too high, causing fuel delivery to be cut
- Reading too low, causing the ECM to underdeliver fuel (Correct answer)
- Functioning normally
- Causing a rich condition
Correct answer: Reading too low, causing the ECM to underdeliver fuel
A MAF reading below the actual airflow causes the ECM to calculate less fuel than needed, resulting in a lean condition.
Question 2: A throttle body has carbon deposits causing the idle to hunt. The BEST repair is to:
- Replace the throttle body assembly
- Clean the throttle body with approved cleaner and relearn the idle (Correct answer)
- Adjust the throttle stop screw to compensate
- Replace the IAC motor
Correct answer: Clean the throttle body with approved cleaner and relearn the idle
Carbon deposits on the throttle body bore and plate can cause erratic idle, which is corrected by cleaning and performing an idle relearn.
Question 3: During a fuel injector leak-down test, one injector causes a fuel pressure drop of 20 psi in 5 minutes while others show only 2–3 psi drop. This means that injector is:
- Clogged and not flowing enough fuel
- Leaking internally and allowing fuel to drip into the intake (Correct answer)
- Functioning within normal parameters
- Sticking open during operation
Correct answer: Leaking internally and allowing fuel to drip into the intake
Excessive pressure drop on one injector during a static leak-down test indicates that injector is not sealing and is leaking fuel.
Question 4: An engine has a rich exhaust odor and poor fuel economy. Long-term fuel trim (LTFT) reads -20%. This indicates:
- The ECM is adding 20% more fuel to compensate for a lean condition
- The ECM has subtracted 20% fuel, indicating an excessive fuel delivery issue (Correct answer)
- The O2 sensors are switching too slowly
- The engine has a vacuum leak
Correct answer: The ECM has subtracted 20% fuel, indicating an excessive fuel delivery issue
A negative LTFT means the ECM is removing fuel to compensate, indicating the base fuel delivery is richer than the commanded mixture.
Question 5: What is the primary purpose of the PCV (Positive Crankcase Ventilation) system?
- To recirculate exhaust gases to reduce NOx emissions
- To vent crankcase pressure and recycle blow-by gases into the intake (Correct answer)
- To supply fresh air to the fuel tank for vapor control
- To pressurize the oil system for better lubrication
Correct answer: To vent crankcase pressure and recycle blow-by gases into the intake
The PCV system draws blow-by gases from the crankcase into the intake manifold to be burned rather than vented to atmosphere.
Question 6: A clogged PCV valve can cause which of the following symptoms?
- A lean condition and increased power
- Oil leaks due to excessive crankcase pressure (Correct answer)
- A rich condition from excess vacuum
- Rough idle from too much intake air
Correct answer: Oil leaks due to excessive crankcase pressure
A blocked PCV valve prevents crankcase pressure relief, which can force oil past gaskets and seals, causing oil leaks.
ASE4 Fuel, Air Induction, and Exhaust Systems 2
A MAF sensor reads 2 g/s at idle but the normal range is 3–7 g/s.
The engine runs lean.
This indicates the MAF sensor is: