CTE Engine Performance & Tuning 3 — Questions and Answers
Question 1: What is the stoichiometric air-fuel ratio (AFR) for gasoline combustion?
- 12.5:1
- 14.7:1 (Correct answer)
- 16.2:1
- 10.3:1
Correct answer: 14.7:1
14.7:1 is the stoichiometric ratio where all fuel and air are theoretically consumed, yielding the best catalyst efficiency.
Question 2: Which ignition system component stores and releases the high-voltage energy needed to fire a spark plug?
- Crankshaft position sensor
- Ignition coil (Correct answer)
- Distributor rotor
- Camshaft position sensor
Correct answer: Ignition coil
The ignition coil transforms battery voltage into the high voltage (20,000–40,000+ V) required to jump the spark plug gap.
Question 3: A student notices that spark plug electrodes are coated with black, sooty carbon deposits. This condition indicates:
- Lean combustion
- Rich combustion or excessive oil burning (Correct answer)
- Normal wear pattern
- Detonation damage
Correct answer: Rich combustion or excessive oil burning
Black carbon fouling results from an overly rich mixture or oil entering the combustion chamber being incompletely burned.
Question 4: What does base ignition timing refer to in engine tuning?
- Timing advance added by the ECU under load
- The fixed timing set when the ECU advance is disabled or disconnected (Correct answer)
- Maximum allowable timing before knock occurs
- Timing retard applied during cold start
Correct answer: The fixed timing set when the ECU advance is disabled or disconnected
Base timing is the initial timing reference set mechanically or by the ECU base map before dynamic advance is applied.
Question 5: Variable valve timing (VVT) systems improve engine performance primarily by:
- Increasing fuel injection pressure at all speeds
- Optimizing valve opening and closing events across the RPM range (Correct answer)
- Reducing exhaust gas recirculation (EGR) flow
- Controlling turbocharger boost pressure
Correct answer: Optimizing valve opening and closing events across the RPM range
VVT adjusts camshaft phasing to optimize power and efficiency at both low and high RPM operating conditions.
Question 6: A student is testing engine compression. Cylinder pressures are 175, 170, 172, and 95 psi. The best next diagnostic step is:
- Replace all spark plugs immediately
- Perform a wet compression test on the low cylinder (Correct answer)
- Check fuel injector balance rates
- Replace the cylinder head gasket without further testing
Correct answer: Perform a wet compression test on the low cylinder
Adding oil to the low cylinder for a wet compression test distinguishes between worn rings (pressure rises) and valve or head gasket issues (pressure stays low).
Question 7: When using an oscilloscope to analyze a crankshaft position (CKP) sensor signal, a missing tooth in the waveform pattern would indicate:
- A shorted ignition coil
- The timing reference point on the reluctor wheel (Correct answer)
- A faulty mass airflow sensor
- Injector pulse width modulation
Correct answer: The timing reference point on the reluctor wheel
Most CKP reluctor wheels have a missing tooth to provide the ECU with a precise TDC reference point for timing calculations.
CTE Engine Performance & Tuning 3
What is the stoichiometric air-fuel ratio (AFR) for gasoline combustion?