Engine Performance
Scope
Engine performance questions test diagnostic reasoning more than memorized specs. Know OBD-II monitors and readiness, DTC structure, freeze frame and live data, fuel trim, misfire diagnosis, and the basic air, fuel, and spark requirements.
Concepts you must own
- DTCs describe a circuit or system, not a part. Confirm the fault with data before replacing anything.
- Fuel trim: short-term is immediate correction, long-term is learned. Positive trim means the PCM is adding fuel to correct a lean condition; negative trim means it is pulling fuel from a rich condition.
- Lean causes: vacuum leaks, low fuel pressure, dirty or under-reporting mass airflow sensor, exhaust leaks ahead of the oxygen sensor. Rich causes: leaking injectors, high fuel pressure, restricted air intake, failing coolant sensor biased cold.
- Misfire: isolate by cylinder with a scan tool cylinder balance or by swapping ignition components. Check spark, fuel, and mechanical condition such as compression and valve sealing.
- EVAP and catalyst monitors need specific drive cycles to run; a cleared code means all monitors reset.
- Vacuum gauge and exhaust backpressure testing still solve problems scanners cannot.
Traps
A P0171 does not mean the oxygen sensor is bad; the sensor is often reporting correctly. Long-term trim beyond about plus or minus 10 percent indicates a real problem. Do not chase a misfire with parts without checking compression on the affected cylinder. Remember that a vacuum leak leans the engine most at idle, while a weak fuel pump leans it most under load; the trim pattern tells you which.
Worked example
Long-term fuel trim is plus 18 percent at idle and drops to plus 5 percent at 2500 rpm, with a rough idle. That pattern is a classic vacuum leak, since manifold vacuum is highest at idle. Smoke test the intake and repair the leak, then confirm trims normalize.
Rule of thumb
Positive fuel trim means lean, negative means rich; stay within about 10 percent either way.