Engine Performance A8 Flashcards

A technician is diagnosing a P0301 (Cylinder 1 Misfire) code. The cylinder 1 spark plug and ignition coil were replaced, but the misfire code returned. What is the NEXT step in diagnosis?

Answer: Perform a cylinder contribution (power balance) test and check cylinder 1 compression and fuel injector operation

After ruling out the ignition components (plug and coil), the next diagnostic step is to systematically evaluate the remaining causes of cylinder-specific misfire: fuel delivery (injector) and mechanical (compression, valve train). A cylinder contribution test quantifies misfire severity, and compression/injector tests identify the specific cause.

A vehicle has a P0442 (EVAP System Small Leak Detected) code. The EVAP system uses a vacuum-based diagnostic monitor. Where is the MOST common source of small EVAP leaks?

Answer: Loose or missing fuel cap or degraded fuel cap seal

The fuel cap is statistically the most common cause of EVAP small leak codes. The rubber seal on the fuel cap deteriorates with age, and caps are frequently not tightened properly. The fuel cap is also the first thing to check because it is the easiest and least expensive to test and replace.

During diagnosis of a no-start condition, a technician confirms fuel pressure is 45 psi (specification 45-55 psi), spark is present on all cylinders, and compression is 150 psi on all cylinders. The engine cranks but will not start. What should be checked NEXT?

Answer: Verify cam timing and confirm the PCM is commanding injector operation at the correct time

With spark, fuel pressure, and compression confirmed as present, the cause of no-start must be in the timing or delivery of these elements. The PCM may not be commanding the injectors at the correct time relative to crank position, or cam timing may be off so the intake valves are not open when injection and spark occur.

An oxygen sensor heater circuit code (P0141 — O2 Sensor Heater Circuit Malfunction, Bank 1, Sensor 2) is stored. What is the PRIMARY purpose of the heater in an oxygen sensor?

Answer: To bring the sensor to operating temperature quickly to allow closed-loop fuel control before the exhaust heats the sensor

The heater in the oxygen sensor rapidly raises the sensor element to operating temperature (approximately 600°F/315°C) during cold start, enabling closed-loop fuel control within 30-60 seconds instead of waiting for exhaust heat alone, which may take several minutes and result in excessive emissions during warm-up.

A vehicle with variable valve timing (VVT) has a P0011 code (Camshaft Position Timing Over-Advanced, Bank 1). What is the MOST likely cause?

Answer: Sludge or low oil pressure preventing the oil control valve (OCV) from returning the phaser to the retarded position

P0011 indicates the cam is more advanced than commanded. In VVT systems using oil pressure to control a camshaft phaser, sludge in the oil passages or an oil control valve stuck in the advance position prevents the phaser from returning to the base (retarded) position when commanded. Low oil pressure is also a contributing factor.

A technician is using Mode 6 data from an OBD-II scan tool to diagnose a marginal catalytic converter. The Mode 6 data shows the catalytic converter monitor is just barely passing. What is Mode 6 data and how is it useful in this diagnosis?

Answer: Mode 6 shows non-continuous monitor test results including the measured values and pass/fail thresholds for each test

OBD-II Mode 6 (also called Mode $06) displays the results of non-continuous diagnostic monitor tests, including the actual measured value, the minimum/maximum threshold for passing, and whether the test passed or failed. This allows technicians to see a marginal converter result with its actual value relative to the threshold.