EVAP System Operation

The enhanced EVAP system is used to limit unburned fuel vapors (hydrocarbons) from escaping into the atmosphere. Since fuel evaporation in the fuel tank is a normal process, it is necessary to accumulate the unburned fuel vapors so they can be purged into the intake manifold for combustion. The EVAP system consists of:
1. EVAP purge solenoid (located under hood towards the center front of dash)
2. EVAP service port (located in the EVAP purge line underhood below the power brake booster identified by a green colored cap)
3. EVAP vent solenoid (inserted into the EVAP canister located underneath the vehicle in front of the fuel tank)
4. Fuel cap
5. EVAP canister (located in front of the fuel tank)
6. Fuel Level Vent Valve (FLVV)
7. Grade Vent Valve (GVV)
8. Fuel tank pressure sensor (pushed into the secondary fuel pump module)
9. Fuel tank
10. EVAP lines and hoses
11. Engine Control Module (ECM) (attached to a bracket on the engine towards the left side of the vehicle)

When fuel from the fuel tank evaporates, the vapors flow though the fuel level vent valve (FLVV), located on the top of the fuel tank, to the EVAP canister. The EVAP canister is an active carbon fiber storage container that collects fuel vapors and prevents them from entering the atmosphere.

The EVAP canister is vented to the atmosphere through the EVAP vent solenoid. The EVAP vent solenoid is a normally open valve that only closes during parts of the EVAP diagnostic test.

When the ECM goes into purge mode (drawing fuel vapors into the intake manifold), it will pulse width modulate the EVAP purge solenoid by grounding the EVAP purge solenoid circuit. As the EVAP purge valve opens and closes, manifold vacuum draws fuel vapors from the EVAP canister.