Evaporative Emissions System: Description and Operation

Vapor Canister TBI Models:

Evaporative Emission (EVAP) Control System:

Fuel Pressure Control Valve:

PURPOSE
The Evaporative Emissions Control System (EECS) limits fuel vapors from escaping into the atmosphere. The EECS transfers fuel vapor from the sealed fuel tank to an activated carbon (charcoal) storage device (vapor canister). The canister will store the vapors until the engine is able to use the extra fuel vapor.

When the engine is able to use the extra fuel vapor, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

The fuel tank is sealed with a fuel cap that is NOT normally vented to the atmosphere. The fuel tank cap has a safety valve which allows for both pressure and vacuum relief.


CONSTRUCTION
The EVAP control system on uses a sealed fuel tank, a basic two tube canister, a thermal vacuum switch and vapor/fuel hoses.


OPERATION
Fuel vapors from the fuel tank are purged and flow into the vapor canister tube labeled "tank" and are absorbed by the carbon. The canister is purged when the engine is able to use the extra fuel vapor. A vacuum source is applied to the vapor canister tube labeled "purge" to draw fresh air through the top of the canister. The air mixes with the vapor and the mixture is drawn into the intake manifold to be consumed in the normal combustion process.


Thermal Vacuum Switch:
On this system a Thermostatic Vacuum Switch (TVS) is installed in the intake manifold coolant passage to sense engine coolant temperature. This TVS has two ports and is located between the canister and the TBI unit.

When the coolant temperature is below 46°C (115°F), the TVS is closed preventing purge of the canister. When the coolant temperature is above 46°C (115°F), the TVS opens, allowing purge of the canister.


Canister Purge Solenoid:
On vehicles with some California Emissions of Tier 1, the TVS is replaced by a control module controlled canister purge solenoid to cycle operation of the canister. The control module cycles the solenoid when the engine coolant temperature is 46°C (115°F) or the short term fuel trim cell is less than 122 counts.