EVAP Purge Valve

Hello. If the PCM controls the duty cycle to purge vapors according to engine RPM, why in the diagram it has a input port vacuum. What's its purpose? I like to know. Thanks in advance.

You know, that's a good question. :blink: I looked over Theory and Operation in SI and couldn't get a good idea.

It's a pilot vacuum supply which is switched by the solenoid to operate the main valve. The large round disc the two purge lines go to contains a diaphragm which the pilot vacuum pulls against to open the main valve.

Depending on where the vacuum is sourced from it may also have some influence on controlling purge flow.