See Figures 1 and 2
The EVAP control system prevents gasoline vapors from escaping into the atmosphere. When fuel evaporates from the fuel tank, the vapors pass through vent hoses or tubes to the carbon-filled EVAP canister. They are temporarily held in the canister until they can be drawn into the intake manifold when the engine is running.
The vapors are drawn into the engine at idle as well as when the engine is operating above idle speed. This system is called a Bi-Level Purge System where there is a dual source of vacuum to remove the vapor from the EVAP canister.
The hoses used in this system are specifically manufactured. If replacement becomes necessary, it is important to use only fuel resistant hose.
The EVAP canister itself is sealed and maintenance-free. It is used on all gasoline-powered vehicles (only) and is mounted to the left frame rail; below the left side of the truck; near the left door. The EVAP canister is serviced as one complete unit (not including the bracket. The canister is purged by the EVAP canister purge solenoid at predetermined times and certain engine operating conditions.
The EVAP canister purge solenoid controls the vacuum that draws the vapors from the canister. The solenoid is controlled by the Single Module Engine Controller (SMEC) for 189-93 models. For 194-96 models, it is controlled by the Powertrain Control Module (PCM). The PCM operates the solenoid by either switching the ground circuit on or off. When energized the solenoid prevents vacuum from reaching the EVAP canister. When not energized, the solenoid allows vacuum to flow to the EVAP canister. The purge solenoid is mounted to a bracket located near the left-rear side of the intake manifold.
The solenoid opens when the engine reaches normal operating temperature and the vehicle is moving.
The EVAP canister is sealed and maintenance-free. However, a periodic inspection of the unit and vent hoses is advisable. Although the factory does not give recommendations here, you should inspect the unit at least every six months or 6000 miles (9600 km). During your inspection, make sure the unit is mounted firmly in its bracket, that it is not cracked and that the vent hoses are connected and not cracked. Correct any problems you may discover, or refer to the removal and installation procedure in Emission Controls if you suspect the unit itself is malfunctioning.