General System Description, EVAP

PURPOSE
As a part of the fuel system, all vehicles are equipped with fuel vapor evaporative emission control systems to meet federal requirements in effect at the time of production. The fuel tank evaporative emission system allows for controlled release of fuel tank pressure through the evaporative emissions canister.

OPERATION
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 build-up of internal fuel tank pressure. 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 expands rapidly when warmed.
- parking after driving over rough roads, washboard or other harsh surfaces, after filling the fuel tank; agitation of fuel increases expansion.
- parking after driving long distances in high-temperature conditions.
- climbing long grades, especially while towing a trailer or while vehicle is fully loaded.

No service is required if these conditions caused the concern. A blocked fuel tank evaporative emission system can cause abnormal fuel tank pressure and must be serviced. Refer to Powertrain Management / System Diagnosis.

A normally functioning fuel tank evaporative emission system will relieve the pressure build-up.

COMPONENTS
Most components of the evaporative emission control system are serviced by a simple nut, bolt or screw for part removal and installation. If push pins are removed, they must be replaced with new pins.

Fuel Tank Evaporative Emission System
This system consists of the following:
- sealed fuel tank and fuel tank filler pipe
- vacuum relief fuel tank filler cap
- evaporative emission valve
- vapor tube and fuel vapor hoses
- evaporative emissions canister

Engine Evaporative Emission System
This system consists of the following:
- evaporative emission canister purge valve
- evaporative emissions canister