i have no experience with it but this is out of the FAQ's and could start you on the right path or at least narrow it down a little :
Monitors the fuel vapor vacuum and pressure in the fuel tank. The system failure occurs when the EVAP running loss monitor detects excessive fuel tank vacuum with the engine running (but not at idle).
Blockages or kinks in EVAP canister tube or EVAP canister purge outlet tube (between fuel tank, EVAP canister purge valve and EVAP canister)
Fuel filler cap stuck closed (no vacuum relief)
Contaminated fuel vapor elbow on EVAP canister
Restricted EVAP canister
CV solenoid stuck open (partially or fully)
Plugged CV solenoid filter
EVAP canister purge valve stuck open
VREF circuit open (harness near FTP sensor, FTP sensor or PCM)
Damaged FTP sensor