CPT Engine Performance & Tuning 2 — Questions and Answers
Question 1: What does a vacuum leak downstream of the throttle body typically cause in engine operation?
- Rich air-fuel mixture
- Lean air-fuel mixture (Correct answer)
- Retarded ignition timing
- Increased fuel pressure
Correct answer: Lean air-fuel mixture
A vacuum leak introduces unmetered air into the intake manifold, leaning out the air-fuel mixture beyond the intended ratio.
Question 2: Which sensor is primarily responsible for enabling closed-loop fuel control in a modern engine management system?
- Mass airflow sensor
- Throttle position sensor
- Oxygen sensor (Correct answer)
- Coolant temperature sensor
Correct answer: Oxygen sensor
The oxygen sensor provides real-time exhaust gas feedback that the ECU uses to continuously correct fuel trim and maintain stoichiometry.
Question 3: When diagnosing a misfire on a specific cylinder, swapping the ignition coil to another cylinder and observing if the misfire follows the coil indicates what?
- A faulty fuel injector
- A defective coil-on-plug unit (Correct answer)
- Worn spark plug wires
- Low compression in that cylinder
Correct answer: A defective coil-on-plug unit
If the misfire moves to the new cylinder after swapping the coil, the coil itself is confirmed as the defective component.
Question 4: What is the primary function of a variable valve timing (VVT) system?
- Increase compression ratio at high RPM
- Optimize cam phasing for power and efficiency across the RPM range (Correct answer)
- Reduce exhaust back-pressure
- Control throttle position electronically
Correct answer: Optimize cam phasing for power and efficiency across the RPM range
VVT advances or retards camshaft timing relative to crankshaft position to balance low-end torque with high-RPM power output.
Question 5: A long-term fuel trim (LTFT) reading of +18% on Bank 1 most likely indicates what condition?
- Rich mixture on Bank 1
- Lean condition on Bank 1 requiring added fuel (Correct answer)
- Faulty MAP sensor
- Clogged catalytic converter
Correct answer: Lean condition on Bank 1 requiring added fuel
A high positive LTFT value shows the ECU is adding significant fuel to compensate for a lean condition, such as a vacuum leak or weak injector.
Question 6: During a cylinder balance test, disabling an injector that causes little or no RPM drop indicates what about that cylinder?
- That cylinder is contributing normally
- That cylinder is already misfiring or not contributing (Correct answer)
- The fuel pressure is too high
- The timing is advanced on that cylinder
Correct answer: That cylinder is already misfiring or not contributing
A cylinder that shows minimal RPM drop when its injector is cut off was already producing little power, pointing to a misfire or mechanical issue.
Question 7: What is the purpose of an exhaust gas recirculation (EGR) valve in an engine?
- Increase combustion temperature for efficiency
- Reintroduce exhaust gases to reduce NOx emissions (Correct answer)
- Provide additional oxygen to the combustion chamber
- Regulate exhaust back-pressure for power
Correct answer: Reintroduce exhaust gases to reduce NOx emissions
EGR dilutes the intake charge with inert exhaust gas, lowering peak combustion temperatures and reducing the formation of nitrogen oxides (NOx).
CPT Engine Performance & Tuning 2
What does a vacuum leak downstream of the throttle body typically cause in engine operation?