[mwebb]

disconnecting EVAP has zero effect on FE
(except for .... see below)
the ECM will use feedback from the 02 or AFR sensors to keep the
air / fuel ratio at 14.64 to 1 on most cars and where ever it wants it on the rest .

an EVAP system that is functioning as designed can not change this

if you disconnect the EVAP system the system will set a DTC for restricted flow and or incorrect vacuum supply to the LDP leak detection pump , assuming the system has a LDP , some do not .

the DTC will suspend some system testing . the CEL / MIL will remain ON until the system is repaired .

the car will leak HC emissions , which is bad , but wasted HC is wasted fuel that could have been used to move the car down the road so

disconnecting EVAP
ultimately
will UN-improve fuel economy
to the OP...
you need not understand this for it to be true .

the EVAP can function at any time
at the pleasure of the ECM not just at start up