Evaporative Emissions System: Description and Operation

PURPOSE
This system is designed to prevent hydro-carbons from escaping into the atmosphere.

OPERATION
The fuel vapors that form above the gasoline's surface within the tank are sent to an activated-charcoal filter for storage. A solenoid valve then injects these vapors into the intake manifold.
The evaporative emissions control system can incorporate a supplementary leak test.
The operating status of the evaporative emissions control system will fall into one of he following three categories:
1. The EVAP canister is empty. Activating the tank ventilation leans out the fuel/ air mixture.
2. The EVAP canister is full. Activating the tank ventilation enriches the fuel/ air mixture.
3. The filling of the EVAP canister corresponds to a stoichiometric mixture ratio. The fuel-air mixture is neither enriched nor lean.








OBD II CHECKS
- Operation (flow) of the fuel tank vent valve.
- Operation of electrical components, in the course of the Comprehensive Components Diagnosis.
Diagnosis of tank ventilation system through difference pressure measurement.
A differences pressure sensor is used for Audi, in order to monitor the function (flow) of tank ventilation system.

Procedure:
1. Measure pressure with valve closed.
2. Then conduct measurement with open valve. The difference of both pressures must exceed a defined limit value, if the system is intact.

SYSTEM COMPONENTS
Evaporative Emission Control Canister
The Evaporative Emission Control (EVAP) Canister collects and stores fuel vapors emitted from the fuel tank when the vehicle is not running. When the engine is running, the stored fuel vapors are sent to the engine via the frequency valve to be burned.

EVAP Canister Purge Regulator Valve N80:

Electrical Circuit - EVAP Canister Purge Regulator Valve N80:

EVAP Canister Purge Regulator Valve N8O
Installation location: adjacent to the air filter housing/strut.
This black-colored unit controls the vapor discharge from the EVAP canister into the intake manifold. It remains closed when no current is applied.

Sensor for EVAP Canister G172
(optional pressure differential sensor)
It gives the signal for the pressure difference test on the engine control module.