Evaporative Emissions System: Description and Operation

Evaporative Emissions Components:

DESCRIPTION and OPERATION
As a part of the fuel system, all vehicles are equipped with fuel vapor Evaporative Emission (EVAP) control systems to meet federal requirements in effect at the time of production.

Evaporative Emission Components:

COMPONENTS

Fuel Tank Evaporative Emission System
- sealed fuel tank and fuel tank filler pipe
- vacuum/pressure relief
- evaporative emission valve

NOTE: Vehicles equipped with a 68 liter (18 gallon) fuel tank require the use of two evaporative emission valves. Vehicles equipped with a 61 liter (16 gallon) fuel tank use only one evaporative emission valve.

- evaporative emission tube shut off valve
- fuel tank vent tube
- evaporative emission tube
- evaporative emissions canister

Engine Evaporative Emission System
- evaporative emission canister purge valve
- evaporative emissions canister
- fuel and vapor return tube

SYSTEM FUNCTIONS
System control and operation are accomplished through four separate basic functions in the system:
- Fill control vent system
- Fuel tank vapor venting and storage system
- Fuel vapor purge system
- Vacuum and pressure relief system

FEATURES
The fuel tank evaporative emission system allows for controlled release of fuel tank pressure through the evaporative emissions canister (EVAP canister). 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 as follows:

- 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, or other harsh surfaces, after filling the fuel tank. Agitation of fuel increases vaporization.

- Climbing long grades, especially while towing a trailer or while fully loaded.

- Parking after driving long distances in high temperature conditions.

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

No service is required if these conditions caused the concern. A blocked fuel tank evaporative emission system however, can cause abnormal fuel tank pressure and must be serviced.