CPT Engine Performance & Tuning 3 — Questions and Answers
Question 1: Which ignition timing condition is most likely to cause engine knock (detonation) under load?
- Timing too far retarded
- Timing too far advanced (Correct answer)
- Cam timing retarded 5 degrees
- Idle timing set at TDC
Correct answer: Timing too far advanced
Overly advanced ignition timing causes the air-fuel mixture to ignite too early, resulting in abnormal combustion pressure that causes knock.
Question 2: A technician finds a crankshaft position sensor signal that drops out at operating temperature. The most likely cause is:
- Faulty PCM
- Heat-related sensor or wiring failure (Correct answer)
- Low battery voltage
- Failed camshaft sensor
Correct answer: Heat-related sensor or wiring failure
Intermittent signal loss that only occurs at operating temperature is a classic symptom of a heat-sensitive sensor or wiring insulation failure.
Question 3: What parameter does the mass airflow (MAF) sensor measure to help the ECU calculate fuel delivery?
- Intake manifold pressure
- Air density and volume flow rate (Correct answer)
- Throttle plate angle
- Exhaust oxygen content
Correct answer: Air density and volume flow rate
The MAF sensor measures the actual mass of air entering the engine per unit time, which the ECU uses as the primary input for fuel injection pulse width.
Question 4: When performing a compression test, a cylinder reading significantly lower than the others that improves greatly with a wet test (adding oil) suggests:
- Leaking head gasket
- Worn or stuck piston rings (Correct answer)
- Bent intake valve
- Burned exhaust valve
Correct answer: Worn or stuck piston rings
Oil temporarily seals worn ring gaps, so a compression increase on the wet test points to ring or cylinder wall wear rather than a valve problem.
Question 5: A faulty coolant temperature sensor (CTS) stuck at a cold reading will cause the ECU to:
- Advance timing and lean the mixture
- Continuously enrich the fuel mixture as if at cold start (Correct answer)
- Disable the EGR system permanently
- Increase idle speed and lean the mixture
Correct answer: Continuously enrich the fuel mixture as if at cold start
The ECU interprets the cold reading as a perpetual cold-start condition and adds extra fuel enrichment, causing a persistent rich running condition.
Question 6: Which scan tool data PID is most useful for evaluating catalytic converter efficiency?
- Short-term fuel trim (STFT)
- Downstream O2 sensor activity vs. upstream O2 sensor activity (Correct answer)
- Manifold absolute pressure (MAP)
- Injector pulse width
Correct answer: Downstream O2 sensor activity vs. upstream O2 sensor activity
A healthy converter shows minimal switching activity on the downstream O2 sensor compared to the constantly switching upstream sensor, indicating efficient oxygen storage.
Question 7: Detonation in a high-performance engine running on pump gasoline can best be prevented by:
- Increasing boost pressure
- Retarding ignition timing or using higher octane fuel (Correct answer)
- Richening the mixture beyond stoichiometry
- Increasing coolant temperature
Correct answer: Retarding ignition timing or using higher octane fuel
Retarding timing reduces the propensity for autoignition, and higher octane fuel resists detonation better under high cylinder pressures.
CPT Engine Performance & Tuning 3
Which ignition timing condition is most likely to cause engine knock (detonation) under load?