Evaporative Emissions System: Description and Operation

As a part of the fuel system, vehicles are equipped with a fuel vapor evaporative emission control system designed to meet Federal and state requirements in effect at the time of production.

The fuel tank evaporative emission system allows for controlled release of fuel tank vapor through the evaporative emissions (EVAP) canister to the engine intake system. Under normal operating conditions, this system will allow sufficient venting to prevent a build-up of internal fuel tank pressure.

Some operating conditions may cause temporary internal fuel tank pressure. In a normally functioning system, pressure could be relieved through vapor venting. Some of these conditions are:

- On warm or hot days, parking the vehicle after filling the fuel tank, the fuel is cool from underground storage and vaporizes rapidly when warmed.
- Parking after driving over rough roads, washboard, etc., after filling the fuel tank. Agitation of fuel increases vaporization.
- Parking after driving long distances in high-temperature conditions with low fuel level.
- Climbing long grades, especially while towing a trailer or while vehicle is fully loaded.

No service is required if these conditions caused the customer concern. A normally-functioning fuel evaporative emission system will relieve the pressure build-up. A blocked fuel evaporative emission system can cause abnormal fuel tank pressure and must be serviced. Refer to Inspection and Verification. Most component parts of the evaporative emission control system are retained by a simple nut, bolt or screw. If push pins are removed, they must be replaced with new push pins.

Fill control and fuel vapor venting is accomplished by the following method:

- A venting line integral to the fuel tank filler pipe extends inside the fuel tank.
- The two evaporative emission valves are mounted in the top panel of the fuel tank and attached by the evaporative emission return tube, the fuel return and brake tube and evaporative emission hose to the evaporative emission canister.
- The evaporative emission valves control fuel tank fill by venting pressure in the fuel tank until the fuel level covers the fill vent opening in the fuel tank filler pipe.
- The vent system is designed to provide an air space, approximately 10 percent of the fuel tank volume, above the fuel level in the fuel tank.
- The air space allows for thermal expansion of fuel and provides clearance between the fuel level and the evaporative emission valve.
- The clearance is sufficient to allow fuel vapor venting through the orifice under most static and dynamic vehicle conditions.

The fuel tank evaporative emission system consists of:

- Sealed fuel tank and fuel tank filler pipe
- Pressure/vacuum relief fuel tank filler cap
- Evaporative emission valve with pressure relief and open flow to vacuum
- Evaporative emission tube and evaporative emission hoses
- Evaporative emissions canister

The engine evaporative emission system consists of:

- Evaporative emission canister purge valve
- Evaporative emissions canister
- Evaporative emission hoses to engine intake system

Since usage of these components depends on the vehicle calibration, refer to the emission calibration label on the front door or door lock pillar. Emission Calibration Number

System control and operation are accomplished through four separate basic functions in the system:

- Fill control venting with fuel tank filler cap off
- Fuel tank vapor venting and storage with fuel tank filler cap on
- Fuel vapor purge systems
- Pressure and vacuum relief system (fuel tank filler cap and evaporative emission valve)