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 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 detect a leak as small as 0.040 inch. The control module uses several tests to determine if the EVAP system is leaking.

This tests for large leaks and blockages in the 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 into 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, DTC P0440 will set.

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, DTC P0442 will set.

After the small leak test has passed, a vacuum is retained in the 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 FTP sensor is monitored for a decrease in EVAP system vacuum. If the vacuum does not decrease to near 0 inches H2O in a calibrated time, DTC P0446 will set.

If the 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), sealing the system, and monitoring the FTP for an increase in vacuum. If the control module detects that EVAP system vacuum increases above a calibrated value, DTC P1441 will set.

(1)	EVAP Canister Purge Solenoid
(2)	EVAP Canister
(3)	Fuel Fill Neck/Fill Cap
(4)	Rollover Valve/Fuel Tank Pressure (FTP) Sensor
(5)	Fuel Tank
(6)	EVAP Canister Vent Solenoid
(7)	Vent Hose/Pipe
(8)	EVAP Vapor Pipe
(9)	EVAP Purge Pipe
(10)	EVAP Service Port

EVAP Canister

The EVAP canister is a sealed unit with 3 ports.

1. Purge pipe
2. Vapor pipe
3. Vent pipe

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 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 Valve

The EVAP vent valve controls fresh airflow into the EVAP canister. It is a normally open valve. 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 and 4.9 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.

Tools Required

•	J 41413 EVAP Pressure/Purge Diagnostic Station

•	J 41416 Ultrasonic Leak Detector

EVAP Pressure/Purge Diagnostic Station

The J 41413 EVAP Pressure/Purge Diagnostic Station is used to help locate leaks in the EVAP system. It provides a clean, dry, regulated supply of nitrogen to pressurize the EVAP system. Refer to the directions on the cart for proper operation.

The J 41415 fuel filler neck adapter is used to adapt the EVAP pressure/purge diagnostic station to the fuel filler neck. Pressurizing the EVAP system at the fuel filler neck allows testing of the filler neck and the fuel fill cap on vehicles with ORVR (Onboard refueling vapor recovery).

The J 41416 Ultrasonic Leak Detector is then used to pinpoint the source of the leak.