Engine Performance & Tuning Flashcards

What does a vacuum leak typically cause in an engine management system?

Answer: Lean air-fuel mixture and rough idle

A vacuum leak introduces unmetered air, causing a lean condition and unstable idle as the ECU struggles to compensate.

Which sensor directly measures the oxygen content in exhaust gases to enable closed-loop fuel control?

Answer: Oxygen (O2) sensor

The oxygen sensor signals the ECU whether combustion is rich or lean, enabling closed-loop fuel trim adjustments.

Long-term fuel trim (LTFT) of +25% most likely indicates:

Answer: A lean running condition requiring more fuel

A high positive LTFT means the ECU is consistently adding fuel to compensate for a persistent lean condition.

What is the primary function of a knock sensor in engine management?

Answer: Detect detonation and signal the ECU to retard timing

The knock sensor detects detonation vibrations and the ECU retards ignition timing to prevent engine damage.

When teaching students about volumetric efficiency (VE), which factor most directly improves VE at high RPM?

Answer: Optimizing intake and exhaust valve timing and port design

Optimized valve timing and port velocity allow the cylinder to fill more completely at high RPM, increasing VE.

A misfire on cylinder 3 that disappears after swapping injectors between cylinders 3 and 5 indicates:

Answer: A clogged or failed fuel injector originally on cylinder 3

When the misfire follows the swapped injector to cylinder 5, the injector itself is confirmed as the faulty component.

In a sequential fuel injection (SFI) system, how does injection timing differ from batch injection?

Answer: Each injector fires independently timed to its cylinder's intake stroke

SFI fires each injector individually just before that cylinder's intake valve opens, improving fuel atomization and efficiency.