The purge flow monitor works continuously and is designed to detect low purge flow caused by a blockage in the purge system or a malfunctioning EVAP canister purge valve.
The basis of the diagnostic is to detect the presence of intake pressure pulses caused by the 10 Hz pulse width modulated control of the EVAP canister purge valve duty (as shown in the purge operation diagram below).
A discrete Fourier transform (DFT) calculation is used to help distinguish these pulses from other noises present in the intake pressure signal.
Scheme 37
Purge operation
