Which PID in a cruise data log indicates a vehicle concern?

• A. MAP
• B. TPS
• C. MAF
• D. IAT

Answer: (C)

Air measurement and fuel metering during cruise rely on the Mass Air Flow reading because it directly tells the ECU how much air is entering the engine. When the engine is cruising, the ECU uses the MAF value to set the correct fuel amount. If the MAF reading is off for the actual engine speed and throttle, the fuel delivery will be mismatched, and the ECU will compensate with fuel trims. In a cruise data log, you’d look for a MAF value that doesn’t track with the RPM and throttle position or that remains out of expected range while the engine maintains steady speed. That kind of mismatch or persistently abnormal MAF signal is a strong indicator of a problem, such as a dirty sensor, a vacuum leak, or a failing MAF sensor, which would explain drivability issues or poor cruise performance.

Other PIDs like manifold pressure, throttle position, or intake air temperature can signal other kinds of issues or conditions, but they don’t provide as direct a measure of how the engine is metering air and fuel during cruise as the Mass Air Flow reading does.