This system prevents the escape of raw fuel vapors (unburned hydrocarbons, or HC) into the atmosphere. When the engine is not running, fuel vapors that build up in the tank flow through a hose to a carbon canister located in the inner fender below the air cleaner. When the engine is running, the vapors in the canister are carried to the intake manifold and burned in the engine.
On vehicles with a Digifant engine management system, the evaporative emission system is completely vacuum operated. The canister is purged of fuel vapors when a vacuum actuated valve is opened and fresh air is drawn into the open bottom of the canister. Vacuum for the valve is generated only at partial throttle openings. The purge air flows to the intake manifold through an orifice that limits the flow. These two features avoid radical air/fuel mixture changes when the canister is purged.
On vehicles with CIS-E fuel injection, the carbon canister is isolated from the intake manifold by a solenoid valve and a frequency valve. The solenoid valve is ON (open) whenever the engine is running. The frequency valve is cycled open and closed by the Motronic control unit to control flow rate depending on coolant temperature and engine speed/load conditions.
On both systems, the vacuum line to the canister connects to the vacuum vent valve. This valve is mounted near the fuel filler and allows vapors to flow to the canister but will close if the vehicle turns over to prevent a liquid fuel leak.
The system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks and check the seal on the gas tank filler cap. Replace the cap if the seal is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If there is a strong smell of raw fuel from under the hood when the engine is not running, test the purge control valve.
- With the engine OFF, disconnect the hoses from both valves and connect a clean length of hose. It should be possible to blow through the frequency valve (normally open) but not the solenoid valve (normally closed).
- With the engine coolant less than 140°F (60°C), disconnect the purge hose from the canister to the frequency valve. With the engine at idle, there should be no vacuum.
- As the engine warms to operating temperature, the solenoid valve should be ON and open. The frequency valve should begin to cycle ON and OFF , open and closed. There will be strong vacuum for about 30 seconds, then little or no vacuum for about 60 seconds.
- If the system does not perform as described, pull back the connector boots so a voltmeter or test light can be connected with the wiring still connected to the valves. If voltage appears at the intervals described, 1 or both of the valves is faulty. If there is no voltage, the wiring or the engine control unit may be faulty.
- Disconnect the top hose from the valve and connect a hand vacuum pump to the small port on top of the valve.
- Disconnect the hose from the carbon canister to the control valve and run the engine at idle to provide vacuum to the bottom port.
- Place a finger over the open port of the valve. When there is vacuum at the bottom port, you should not feel any vacuum at the open port.
- Draw a vacuum on the small top port. The valve should open and you should feel vacuum at the open port.