The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge valve ON open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve OFF open, fresh air will be drawn through the valve and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge valve into the intake manifold to be consumed during normal combustion. The EVAP system is capable of detecting a leak as small as 0.04 inch. The control module uses several tests to determine if the EVAP system is leaking.

This tests for large leaks and blockages in the evaporative emission (EVAP) system. The control module will command the EVAP vent valve ON closed and command the EVAP purge valve ON open with the engine running, allowing engine vacuum in the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. The control module then commands the EVAP purge valve OFF closed, sealing the system and monitors the vacuum level for decay. If the control module does not detect that the predetermined vacuum level was achieved, or the vacuum decay is more than a calibrated level on 2 consecutive tests, a DTC sets.

If the large leak test passes, the control module will test for small leaks by continuing to monitor the fuel tank pressure (FTP) sensor for a change in voltage over a period of time. If the decay rate is more than a calibrated value, the control module will rerun the test. If the test fails again, a DTC sets.

After the small leak test has passed, a vacuum is retained in the evaporative emission (EVAP) system. The control module tests for a restricted vent path by commanding the purge valve OFF closed and the vent valve OFF open. The fuel tank pressure (FTP) sensor is monitored for a decrease in EVAP system vacuum. If the vacuum does not decrease to near 0 inch H2O in a calibrated time, a DTC sets.

If the evaporative emission (EVAP) purge valve does not seal properly, fuel vapors could enter the engine at an undesired time causing driveability concerns. The control module tests for this by commanding the EVAP purge valve OFF closed and vent valve OFF OPEN, and monitors the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that EVAP system vacuum increases above a calibrated value, a DTC sets.

The evaporative emission (EVAP) system consists of the following components:

EVAP Canister

The EVAP canister is a sealed unit with 3 ports.

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

EVAP Purge Solenoid Valve

The EVAP purge valve controls the flow of vapors from the EVAP system to the intake manifold. This normally closed valve is pulse width modulated by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

EVAP Vent Solenoid Valve

The EVAP vent valve controls fresh airflow into the EVAP canister. EVAP valve is normally open. The control module will command the valve closed during some EVAP tests, allowing the system to be tested for leaks.

Fuel Tank Pressure Sensor

The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.7 volts. As fuel tank pressure increases, FTP sensor voltage decreases, high pressure = low voltage. As fuel tank pressure decreases, FTP voltage increases, low pressure or vacuum = high voltage.

EVAP Service Port

The EVAP service port is located in the EVAP purge pipe between the EVAP purge valve and the EVAP canister. The service port is identified by a green colored cap.

The Evaporative Emission System Tester is used to help locate leaks in the evaporative emission (EVAP) system. The EEST provides a clean, dry, regulated supply of nitrogen to pressurize the EVAP system. It also provides smoke to help in locating the leak source. Refer to the directions on the cart for proper operation.

The Fuel Tank Cap Adapter is used to adapt the to the fuel filler neck. Pressurizing, or inducing smoke to the EVAP system at the fuel filler neck allows testing of the filler neck and the fuel fill cap on vehicles with onboard refueling vapor recovery (ORVR).

The High Intensity White Light is used to help locate the leak source.

This vehicle is equipped with an On Board Refueling Vapor Recovery System (ORVR). This system has developed and equipped to meet enhanced evaporative emission (EVAP) control requirements during vehicle moving, parking, and refueling. The ORVR system operates in the following manner:

The ORVR system operates by the liquid trap, liquid seal, system which ensures long-term durability. The ORVR system has been designed with the following functional features:

This vehicle is also equipped with an On Board Diagnostic II (OBD II) system. This system identifies failures or malfunctions of the ORVR system and warns the driver through the malfunction indicator lamp (MIL) on the instrument cluster.

The ORVR system requires no special refueling procedures and maintenance.