There is normally vapor pressure in the fuel tank, that is part of the emission control system. The usual cause of the check engine light is a loose fuel cap, but you can check the evaporative emission control system by the following:
1) Warm up engine to normal operating temperature and keep idling
5 min. or more.
2) Disconnect purge hose from EVAP canister when engine is running
at idle speed.
3) Place finger against the end of disconnected hose and check
that vacuum is not felt but vibration is felt there when engine is
running at idle speed.
4) Also check that vacuum is felt when engine speed is increased
to higher than about 2,000 r/min.
5) If the above check result is not satisfactory, check vacuum passage,
hoses, EVAP canister purge valve, wire harness and ECM
Also check the Tank Pressure Control Valve as follows:
1) Remove tank pressure control valve installed by the EVAP canister.
2) Air should pass through valve smoothly from fuel tank side
(black side of tank pressure control valve) to orange side when
3) From orange side, even when blown softly, air should come out
of black side.
4) If air doesn’t pass through valve in step 2) or hard blow is required
in step 3), replace tank pressure control valve.