Mark is correct, the throttle being held open is intended to ensure some airflow and reduce hydrocarbon (HC) and carbon monoxide (CO) under a lean burn condition that can occur with the throttle plate closed.So Cal Mark wrote:throttle dashpots do not control or affect Nox. Nox is formed at high cylinder temps, not during decel.
My memory failed me and I didn't check my facts. Either way, it is a system intended to reduce emissions.