Electrical Systems & Emissions Control Flashcards

What is the primary purpose of the exhaust gas recirculation (EGR) system?

Answer: Reduce NOx by lowering combustion temperatures

Inert exhaust gas dilutes the intake charge, lowering peak combustion temperature and NOx formation.

On an OBD II vehicle with a healthy catalytic converter, how should the downstream oxygen sensor signal look compared to the upstream sensor?

Answer: Relatively steady with little switching

A working converter stores oxygen and smooths fluctuations, so the downstream sensor stays fairly steady.

A zirconia oxygen sensor reading a steady 0.85 volts indicates what condition?

Answer: Rich exhaust

Zirconia sensors produce high voltage (above about 0.45 V) when exhaust oxygen is low, meaning rich.

What does the EVAP purge valve do when commanded open by the PCM?

Answer: Draws stored fuel vapor from the charcoal canister into the intake

Engine vacuum pulls vapors from the canister into the intake to be burned when the purge valve opens.

Which component is the vent valve's partner in sealing the EVAP system during a leak test?

Answer: The purge valve

With both the purge and vent valves closed, the PCM can monitor tank pressure or vacuum to detect leaks.

What does a long-term fuel trim of +20% at idle that drops near 0% at 2,500 rpm most likely indicate?

Answer: A vacuum leak

Unmetered air from a vacuum leak has the largest effect at idle and becomes insignificant as airflow increases.

What is the function of the positive crankcase ventilation (PCV) valve?

Answer: Routes blow-by gases into the intake to be burned

The PCV system removes blow-by vapors from the crankcase and burns them instead of venting them to the air.