Car Tuning Fuel and Ignition Management 2 — Questions and Answers
Question 1: What does a wideband O2 sensor measure that a narrowband sensor cannot?
- Exact air-fuel ratio across a wide lambda range (Correct answer)
- Exhaust gas temperature only
- Manifold absolute pressure
- Fuel injector pulse width
Correct answer: Exact air-fuel ratio across a wide lambda range
A wideband O2 sensor can accurately measure lambda from approximately 0.7 to 32, whereas a narrowband only indicates rich or lean near stoichiometry.
Question 2: When tuning a turbocharged engine, why is ignition timing typically reduced at high boost levels?
- To prevent detonation caused by higher combustion temperatures and pressures (Correct answer)
- To increase fuel economy at wide-open throttle
- To reduce exhaust gas recirculation
- To lower idle speed automatically
Correct answer: To prevent detonation caused by higher combustion temperatures and pressures
Higher boost increases charge density and temperature, making the mixture more prone to knock, so timing must be retarded to keep combustion safe.
Question 3: What is 'closed-loop' fuel control?
- The ECU uses O2 sensor feedback to correct fuel trims in real time (Correct answer)
- The fuel system recirculates unused fuel back to the tank
- Injectors are controlled by a fixed duty cycle map
- The throttle body closes at idle to reduce emissions
Correct answer: The ECU uses O2 sensor feedback to correct fuel trims in real time
In closed-loop operation the ECU reads the O2 sensor and adjusts short-term fuel trims to maintain the commanded air-fuel ratio.
Question 4: A tuner notices positive long-term fuel trims (LTFT) of +18% at idle. What does this most likely indicate?
- A vacuum leak or lean condition the ECU is compensating for (Correct answer)
- The engine is running rich and the ECU is adding fuel
- Ignition timing is too advanced
- The MAP sensor is reading too high
Correct answer: A vacuum leak or lean condition the ECU is compensating for
Positive LTFTs mean the ECU is adding fuel to correct a lean condition, commonly caused by unmetered air entering through a vacuum leak.
Question 5: What is the purpose of a fuel pressure regulator in a return-style fuel system?
- To maintain a constant pressure differential between fuel rail and intake manifold (Correct answer)
- To increase injector duty cycle at high RPM
- To filter debris from entering the injectors
- To measure volumetric flow of fuel consumed
Correct answer: To maintain a constant pressure differential between fuel rail and intake manifold
A manifold-referenced fuel pressure regulator keeps the pressure drop across injectors constant regardless of boost or vacuum, ensuring accurate fuel delivery.
Question 6: Which parameter most directly determines how much fuel an injector delivers per event?
- Pulse width (injector open time in milliseconds) (Correct answer)
- Fuel rail temperature
- Throttle position sensor voltage
- Camshaft duration
Correct answer: Pulse width (injector open time in milliseconds)
Injector pulse width is the duration the injector solenoid stays open, directly controlling the volume of fuel injected per cycle.
Question 7: What happens to combustion efficiency if ignition timing is too retarded (too late)?
- Peak cylinder pressure occurs too late, reducing power and increasing exhaust temperatures (Correct answer)
- The engine detonates due to premature ignition
- Fuel economy improves significantly at all loads
- Volumetric efficiency increases at low RPM
Correct answer: Peak cylinder pressure occurs too late, reducing power and increasing exhaust temperatures
Retarded timing pushes peak pressure past TDC, wasting energy as heat and raising exhaust gas temperatures while reducing torque output.
Car Tuning Fuel and Ignition Management 2
What does a wideband O2 sensor measure that a narrowband sensor cannot?