Engine Performance A8 Flashcards

A scan tool shows a P0300 random misfire code along with P0301, P0303, P0305 (cylinders 1, 3, 5). All affected cylinders are on the same bank. Technician A says a faulty fuel injector on each cylinder is the cause. Technician B says a restricted fuel injector on that bank or a vacuum leak at the intake manifold gasket is more likely. Who is correct?

Answer: Technician B only

When misfires are confined to one bank, the cause is most likely shared by that bank — such as a vacuum leak at the intake manifold gasket (causing a lean condition), a failed coil pack serving that bank, or a bank-specific issue. Multiple simultaneous injector failures on one bank are highly unlikely.

A vehicle has a P0171 (System Lean, Bank 1) code. The long-term fuel trim (LTFT) for Bank 1 is +22%. The MOST likely cause is:

Answer: A vacuum leak or MAF sensor underreporting air mass on Bank 1

A high positive LTFT means the ECM is adding significant fuel because it sees a lean condition. A vacuum leak introduces unmetered air, or a dirty/faulty MAF sensor underreports airflow, both of which cause the system to run lean. If both banks showed high positive LTFT, a fuel pressure issue would be more likely.

An oscilloscope is connected to the primary circuit of an ignition coil. The waveform shows a ringing pattern with multiple oscillations after the spark line collapses. This indicates:

Answer: Normal coil operation — oscillations confirm coil energy is dissipating correctly

The ringing or oscillations visible after the spark line collapses on an ignition primary waveform are normal. They represent the remaining energy in the coil dissipating as the magnetic field collapses. The number and amplitude of oscillations indicate coil health — few or no oscillations suggest a weak coil.

A vehicle with a returnless fuel system has a P0087 (Fuel Rail Pressure Too Low) code. With the engine running, fuel rail pressure is 15 psi below specification. The technician's FIRST test should be:

Answer: Check for a clogged fuel filter and verify fuel volume delivery at the rail

In a returnless system, a clogged fuel filter is a common cause of low pressure. Volume testing (flow test) determines if the pump is producing adequate flow. A pump may maintain pressure at idle but fail under load if partially clogged. Filter restriction should be ruled out before condemning the pump.

During an evaporative emission system test, a scan tool shows the EVAP monitor did not complete after two drive cycles. The technician checks and finds the fuel cap passes a pressure test. The NEXT step is:

Answer: Perform a smoke test on the EVAP system to check for leaks

If the EVAP monitor fails to complete and the fuel cap is good, the system should be tested for leaks using an EVAP smoke machine. Small leaks that do not set a code can prevent the monitor from completing. A smoke test quickly identifies leaks in hoses, canisters, and the vent or purge valves.

A vehicle has a complaint of poor fuel economy and sluggish performance. A scan tool shows intake cam timing is retarded 12 degrees from commanded position on Bank 1. The MOST likely cause is:

Answer: Low oil pressure, a clogged VVT solenoid screen, or a faulty VVT actuator

Variable valve timing (VVT) systems use oil pressure to advance or retard cam timing. If cam timing is retarded from commanded position, the actuator is not receiving sufficient oil pressure or the VVT oil control solenoid is clogged or faulty. A clogged solenoid screen is a common cause and is checked first.