Evaporative Emissions System: Description and Operation

Vapor Canister Except Altitude:

Evaporative Emission (EVAP) Control System:

PURPOSE
The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere by storing the vapors in an activated carbon (charcoal) storage device (vapor canister) 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.


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 fuel vapor and the mixture is drawn into the intake manifold to be consumed in the normal combustion process.
While the canister is purging, fresh air is drawn in through the air inlet at the top of the canister. Air flows to the bottom of the canister and forces the vapors out the "purge" tube.

Canister Purge Solenoid
California equipped EVAP systems use an electrically controlled solenoid to cycle the vacuum to the purge canister. The control module cycles the solenoid when engine coolant temperature is greater than 113°F (45°C) or the short term fuel trim cell is less than 122 counts.

Tank Pressure Control Valve (TPCV)
The TPCV, used only on altitude applications, allows vapors to flow from the fuel tank into the system. When fuel tank pressure exceeds the spring pressure on the valve diaphragm the valve opens and allows vapors to enter the canister or go directly to the engine when purge is enabled. The TPCV is located in or near the fuel tank.

Fuel Pressure Control Valve:

Fuel Tank Cap
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.

Canister Location
The canister is located in the forward part of either the left or right front fender well next to the radiator support.