CPT CPT Data Analysis & Interpretation 2

Question 1

What does a long-term fuel trim (LTFT) value of -15% on both banks indicate about the fuel system?

Accepted Answer

A rich running condition requiring the ECU to remove fuel

Answer

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?

Accepted Answer

Normal power band characteristics related to RPM

Answer

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?

Accepted Answer

Underfueling leading to lean misfire and reduced power

Answer

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?

Accepted Answer

Cam actuator position vs. commanded position

Answer

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?

Accepted Answer

A restriction in the intake tract reducing volumetric efficiency

Answer

On a NA engine at WOT, MAP should approach atmospheric pressure (~14.7 psi); a low MAP indicates an intake restriction reducing cylinder filling.

CPT - Certified Performance Technician Practice Test

CPT - Certified Performance Technician Practice Test

CPT CPT Data Analysis & Interpretation 2