Evaporative Emission Control System Overview
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 charcoal canister absorbs and stores the fuel vapors.Excess pressure is vented through the vent line and EVAP vent solenoid to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the PCM commands the EVAP purge solenoid “on”, allowing engine vacuum to be applied to the canister. With the EVAP vent solenoid “off”, fresh air is drawn through the vent solenoid and the vent line to the 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 solenoid into the intake manifold where it is consumed during combustion. The PCM uses several tests to determine if the EVAP system is leaking.
EVAP System Components
A typical EVAP system consists of the following components: * EVAP Charcoal Canister * EVAP Canister Purge Valve * EVAP Canister Vent Valve * EVAP Service Port (has a green cap) * EVAP Vapor Vent Pipe * Fuel Fill Neck/Fill Cap * Fuel Tank * Rollover Valve with integral Fuel Pressure Sensor (FTP) * Vent Hose/Pipe * EVAP Vapor Pipe
Large Leak Test
The Large Leak diagnostic tests for large leaks and blockages in the EVAP system. With the engine running, the PCM commands the EVAP vent solenoid “on” and commands the EVAP purge solenoid “on”, allowing engine vacuum into the EVAP system. The PCM monitors the fuel tank pressure (FTP) sensor signal to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. Next, the PCM commands the EVAP purge solenoid “off”, to seal the system, and monitors the vacuum level for decay. If the PCM does not detect that the predetermined vacuum level is achieved, or the vacuum decay rate is more than a calibrated level on two consecutive tests, DTC P0455 will be set.
Small Leak Test
If the large leak test passes, the PCM will test for small leaks by continuing to monitor the FTP sensor for a change in FTP sensor signal 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 be set.
Engine Off Natural Vacuum Test (2002-03 Truck Models <8500 GVW)
The Engine Off Natural Vacuum (EONV) test is the small-leak detection diagnostic for the EVAP system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the key “off”. Because of this design, it may be normal for the PCM to remain active for up to 40 minutes after the engine is turned “off”. This is important to remember when performing a parasitic draw test on vehicles equipped with the EONV test.The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, and then starts to drop. The EONV test relies on this temperature change and its corresponding pressure change to determine if a leak is present in the EVAP system.The EONV test is designed to detect leaks as small as 0.51 mm (0.020 inches). The test can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 inches) leak is present, often little or no pressure or vacuum is observed. If the test fails, DTC P0442 will be set.
Canister Vent Restriction Test
If the EVAP Vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The PCM tests this possibility as discussed next. The PCM commands the EVAP purge solenoid “on”, commands the EVAP vent solenoid “off”, and monitors the FTP sensor signal for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will be set.
Purge Solenoid Leak Test
If the EVAP purge solenoid does not seal properly, fuel vapors could enter the engine at an undesired time, causing driveability concerns. The PCM tests for this possibility as follows. The PCM commands the EVAP purge solenoid “off” and the vent solenoid “on”, to seal the EVAP system, and then monitors the FTP for an increase in vacuum. If the PCM detects that the EVAP system vacuum increases more than a calibrated value, DTC P0496 will be set.
Check Gas Cap Message
The PCM sends a class 2 message to the driver information center (DIC) to illuminate the Check Gas Cap message when any of the following occur: * The Large Leak Test failed due to a malfunction in the EVAP system * The Small Leak Test failed due to a malfunction in the EVAP system
EVAP System Monitor Operation
The EVAP System Monitor is a PCM diagnostic that runs once per trip that monitors the EVAP system in order to detect a loss of system integrity or a leak in the system (leaks that range in size from 0.020" to 0.040" in diameter). It also checks the ability of the EVAP system to flow fuel vapors in the system.The PCM controls the Purge solenoid to open and close the path of vacuum to the canister at off-idle speeds. The PCM monitors changes in the fuel tank pressure (FTP) sensor signal with the EVAP vent valve closed. If the PCM detects the system cannot hold proper vacuum, a trouble code is set and the current engine conditions are stored in Freeze Frame. Refer to the EVAP System Graphic as needed.
Possible Causes of an EVAP System Failure
* Cracks, leaks or disconnected hoses in the fuel vapor lines, components, plastic connectors or lines * Charcoal canister is damaged or saturated with fuel (due to EVAP vent valve problems) * EVAP service port is leaking * Fuel filler cap (gas cap) is loose or missing * PCM has failed
EVAP Purge Conditions
The PCM purges the canister under these conditions: * After the engine has reached operating temperature * At wide open or part throttle (as long as the engine is not overheated) * The engine can be in open or closed loop Fuel Control in a purge event
MIL Operation, How to Clear History Trouble Codes
If the EVAP Monitor detects a fault during a drive cycle, it will set a pending code and store the current vehicle conditions in Freeze Frame. If it detects the fault for two consecutive trips, the MIL is activated and a code is set. The MIL will remain on for more than one trip, but will go out if conditions that caused the EVAP Monitor to fail do not reappear on three consecutive trips. After the MIL is off, the trouble code will be erased after 40 consecutive trips if the fault does not reappear.
Freeze Frame Data
The PCM stores the current engine operating conditions at the time a trouble code is set in a special portion of memory called Freeze Frame. This important information can help pinpoint the cause of trouble code. It can be retrieved with an OBD II compatible Scan Tool to help diagnose the problem.
EVAP System Repair Verification
An example of how to drive a vehicle to verify the repair of a trouble code related to the EVAP System Monitor on these vehicle applications is shown in the Graphic below.