CTE Engine Performance & Tuning 2 — Questions and Answers
Question 1: What does a vacuum leak typically cause in an engine management system?
- Rich air-fuel mixture and increased power
- Lean air-fuel mixture and rough idle (Correct answer)
- Excessive fuel pressure and flooding
- Retarded ignition timing only
Correct 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.
Question 2: Which sensor directly measures the oxygen content in exhaust gases to enable closed-loop fuel control?
- Mass airflow (MAF) sensor
- Manifold absolute pressure (MAP) sensor
- Oxygen (O2) sensor (Correct answer)
- Throttle position sensor (TPS)
Correct answer: Oxygen (O2) sensor
The oxygen sensor signals the ECU whether combustion is rich or lean, enabling closed-loop fuel trim adjustments.
Question 3: Long-term fuel trim (LTFT) of +25% most likely indicates:
- A rich running condition
- A lean running condition requiring more fuel (Correct answer)
- Advanced ignition timing
- A faulty coolant temperature sensor
Correct 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.
Question 4: What is the primary function of a knock sensor in engine management?
- Measure intake air temperature
- Detect detonation and signal the ECU to retard timing (Correct answer)
- Monitor exhaust backpressure
- Control variable valve timing
Correct 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.
Question 5: When teaching students about volumetric efficiency (VE), which factor most directly improves VE at high RPM?
- Increasing coolant temperature
- Optimizing intake and exhaust valve timing and port design (Correct answer)
- Reducing engine displacement
- Decreasing compression ratio
Correct 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.
Question 6: A misfire on cylinder 3 that disappears after swapping injectors between cylinders 3 and 5 indicates:
- A faulty coil on cylinder 3
- A clogged or failed fuel injector originally on cylinder 3 (Correct answer)
- A compression loss in cylinder 3
- A wiring harness fault to the ECU
Correct 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.
Question 7: In a sequential fuel injection (SFI) system, how does injection timing differ from batch injection?
- All injectors fire simultaneously twice per revolution
- Each injector fires independently timed to its cylinder's intake stroke (Correct answer)
- Injectors fire in groups of two simultaneously
- Injection timing is fixed and not adjustable by the ECU
Correct 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.
CTE Engine Performance & Tuning 2
What does a vacuum leak typically cause in an engine management system?