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 powertrain control module (PCM) 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 PCM uses several tests to determine if the EVAP system is leaking.
This tests for large leaks and blockages in the EVAP system. The PCM 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 PCM 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 PCM then commands the EVAP purge valve OFF (closed), sealing the system and monitors the vacuum level for decay. If the PCM does not detect that the predetermined vacuum level was acheived, or the vacuum decay is more than a calibrated level on 2 consecutive tests, a DTC P0440 will set.
If the large leak test passes, the PCM 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 PCM will rerun the test. If the test fails again, a DTC P0442 will set.
If the EVAP vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The PCM tests this by commanding the EVAP purge valve ON (OPEN) and commanding the EVAP vent valve OFF (OPEN) and monitoring the FTP sensor for an increase in vacuum. If vacuum increases more than a calibrated value, 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 PCM 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 PCM detects that EVAP system vacuum increases above a calibrated value, DTC P1441 will set.
The EVAP system contains the following components;
The EVAP canister is a sealed unit with 3 ports.
The canister (3) is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the PCM determines that the vapor can be consumed in the normal combustion process.
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 PCM 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.
The EVAP vent valve controls fresh airflow into the EVAP canister. It is a normally open valve. The PCM will command the valve closed during some EVAP tests, allowing the system to be tested for leaks.
The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The PCM provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the PCM that can vary between 0.1-4.9 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).
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 EVAP Pressure/Purge Diagnostic Station (2) 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 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 onboard refueling vapor recovery (ORVR).
The J 41416 ultrasonic leak detector is then used to pinpoint the source of the leak.
