Fuel, Air Induction, and Exhaust Systems Flashcards

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:

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.

A throttle body has carbon deposits causing the idle to hunt. The BEST repair is to:

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.

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:

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.

An engine has a rich exhaust odor and poor fuel economy. Long-term fuel trim (LTFT) reads -20%. This indicates:

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.

What is the primary purpose of the PCV (Positive Crankcase Ventilation) system?

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.

A clogged PCV valve can cause which of the following symptoms?

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.