Exhaust Gas Recirculation (EGR) reduces NOx emissions by what mechanism, and what are common faults that impede EGR operation?

• A. EGR lowers combustion temperatures by recirculating a portion of exhaust gas back into the intake. Common faults: clogged passages, stuck EGR valve, faulty control solenoid/valve, vacuum leaks, or incorrect pressures.
• B. EGR increases combustion temperatures to improve power. Common faults: overcooling, excessive vacuum, and loose hoses.
• C. EGR has no effect on NOx; common faults are unrelated to emissions control.
• D. EGR reduces fuel economy by blocking exhaust gas entirely.

Answer: (A)

EGR lowers NOx by reducing combustion temperatures. It accomplishes this by routing a portion of the exhaust gas back into the intake, where it mixes with the incoming air. The recirculated gas is largely inert and dilutes the air–fuel mixture, which lowers the peak flame temperature and slows heat release. With cooler combustion, thermal NOx formation drops, so overall NOx emissions decrease.

Common faults that impede EGR operation include clogged passages that restrict flow, a stuck EGR valve that can’t open or close, a faulty control solenoid or valve that fails to actuate, vacuum leaks in vacuum-operated systems, and incorrect pressures or faulty sensing that prevent proper EGR timing or amount. When these faults occur, EGR can’t effectively reduce NOx, and emissions performance suffers.