Originally Posted by
zip439
" Does anybody know the purpose of the canister shutoff valve? "
The shut off valve is used to test the pressure in the tank. It's normal position is open. It will only be closed when the PTCM wants to test the gas tank and EVAP system for leaks.
You did a much better job of explaining this than the manual.
I have my charcoal canister just sitting and letting it air out. I will test the valve and make sure it is working correctly.