Engine Performance A8 Flashcards

A scan tool displays a long-term fuel trim (LTFT) of +18% at idle. What does this value indicate and what is the MOST likely cause?

Answer: The engine is running lean; the PCM has added 18% more fuel to compensate for a lean condition

Positive long-term fuel trim indicates the PCM has added fuel beyond the base fuel map to correct a lean condition. LTFT of +18% (typical acceptable range is ±10%) indicates the PCM is compensating significantly for a lean-running condition, pointing to a vacuum leak, MAF sensor problem, or lean injector(s).

A misfiring condition is present on cylinders 1, 3, and 5 only (all odd-numbered cylinders). These cylinders share a common intake manifold runner. What is the MOST likely cause?

Answer: A vacuum leak or restricted intake affecting only the odd-cylinder bank's intake runners

When multiple cylinders on the same bank or sharing a common intake manifold path all misfire, the cause is almost always related to the shared component — in this case, the intake manifold runners feeding cylinders 1, 3, and 5. A leak, restriction, or gasket failure affecting only those runners would cause misfires on exactly those cylinders.

During an exhaust gas analysis, a technician finds elevated hydrocarbon (HC) readings with normal CO and CO2. What does this pattern most likely indicate?

Answer: Engine misfire or a cylinder not contributing to combustion

Elevated HC with normal CO and CO2 indicates unburned fuel is exiting the exhaust without combustion occurring in at least one cylinder. This is the signature of engine misfire — the fuel passes through the cylinder unburned (high HC) without the CO or CO2 changes that would occur during rich combustion.

A vehicle fails an I/M (inspection/maintenance) emissions test with excessive NOx emissions. The EGR system is suspected. How does EGR help reduce NOx emissions?

Answer: EGR introduces inert exhaust gas to dilute the air/fuel mixture, lowering combustion temperatures that produce NOx

NOx (oxides of nitrogen) forms when combustion temperatures exceed approximately 2500°F. EGR dilutes the intake charge with inert exhaust gas, which reduces oxygen content and slows combustion, lowering peak cylinder temperatures. Lower temperatures prevent the nitrogen-oxygen reaction that forms NOx.

A Mass Air Flow (MAF) sensor reads 5.8 grams per second at idle. The specification for idle is 3.0-5.0 g/s. What are the likely drivability symptoms associated with a high MAF reading?

Answer: Rich running condition — the PCM delivers too much fuel based on the overstated airflow reading

A MAF sensor reading higher than actual airflow causes the PCM to calculate more fuel is needed than is actually required. The engine runs rich — more fuel is injected than the actual aircharge can support. Symptoms include black smoke, poor fuel economy, strong fuel odor, and negative fuel trims.

A catalytic converter efficiency test using a scan tool shows the downstream O2 sensor switching almost as fast as the upstream O2 sensor. What does this indicate?

Answer: The catalytic converter has failed and is no longer effectively storing and releasing oxygen

A properly functioning catalytic converter stores and releases oxygen, dampening the oscillating signal of the upstream O2 sensor. The downstream sensor should show a relatively steady voltage in the 0.6-0.7V range. When the downstream sensor switches rapidly (like the upstream), the converter's oxygen storage capacity has been exhausted and it is no longer processing emissions effectively.