CPT CPT Data Analysis & Interpretation 2 — Questions and Answers
Question 1: What does a long-term fuel trim (LTFT) value of -15% on both banks indicate about the fuel system?
- A vacuum leak on one side of the engine
- A rich running condition requiring the ECU to remove fuel (Correct answer)
- A lean condition on bank 2 only
- Normal adaptive learning values
Correct answer: A rich running condition requiring the ECU to remove fuel
Negative LTFT means the ECU has learned to reduce fuel delivery because the engine consistently runs rich.
Question 2: During a WOT pull on a chassis dyno, the horsepower curve drops sharply after a peak but torque remains relatively stable. What does this typically indicate?
- Normal power band characteristics related to RPM (Correct answer)
- A failing torque converter
- Brake drag on the dyno rollers
- Low engine oil pressure
Correct answer: Normal power band characteristics related to RPM
HP = Torque × RPM / 5252; stable torque with falling HP at high RPM is mathematically normal as the power formula's benefit from RPM is offset by torque decline.
Question 3: A technician notices mass airflow (MAF) sensor readings that are lower than expected for a given throttle opening. What performance symptom would this most likely cause?
- Overfueling and rich black smoke
- Underfueling leading to lean misfire and reduced power (Correct answer)
- Excessive ignition advance
- Transmission slipping
Correct answer: Underfueling leading to lean misfire and reduced power
If the MAF underreports airflow, the ECU will inject less fuel than needed, creating a lean mixture and potential misfire.
Question 4: Which scan tool PID would a CPT technician use to verify that a variable valve timing (VVT) system is responding correctly to load demands?
- Cam actuator position vs. commanded position (Correct answer)
- Coolant temperature vs. ambient temperature
- Battery voltage vs. alternator output
- MAP vs. barometric pressure
Correct answer: Cam actuator position vs. commanded position
Comparing actual cam actuator position to the commanded position verifies that the VVT solenoid and oil control valve are functioning correctly.
Question 5: On a data log, a technician observes that throttle position is at 100% but MAP sensor reads only 9 psi on a naturally aspirated engine. What does this suggest?
- A restriction in the intake tract reducing volumetric efficiency (Correct answer)
- Normal wide-open throttle operation
- A boost leak in the turbocharger system
- Excessive exhaust back pressure from a clogged DPF
Correct answer: A restriction in the intake tract reducing volumetric efficiency
On a NA engine at WOT, MAP should approach atmospheric pressure (~14.7 psi); a low MAP indicates an intake restriction reducing cylinder filling.
Question 6: What is the significance of comparing 0-60 mph times against trap speed at the quarter-mile in performance testing?
- Trap speed reflects total power while 0-60 indicates launch/traction efficiency (Correct answer)
- They measure the same performance attribute
- Trap speed is irrelevant to engine output
- 0-60 time is more accurate on wet tracks
Correct answer: Trap speed reflects total power while 0-60 indicates launch/traction efficiency
Trap speed is directly related to peak power output while 0-60 time reflects launch technique, traction, and gearing efficiency.
CPT CPT Data Analysis & Interpretation 2
What does a long-term fuel trim (LTFT) value of -15% on both banks indicate about the fuel system?