1. Early Type Description
Early Type (Non-Intrusive) EVAP System Overview
There are a variety of EVAP systems in use with different monitoring strategies. It is essential that the EVAP system be correctly identified before beginning diagnosis. The Repair Manual is the best source for this information. The following information covers the different systems.
The first system described is the Early Type (Non-Intrusive) EVAP System. Refer to the Applicable Vehicles chart for applicability information.
Purge Operation
When the engine has reached predetermined parameters (closed loop, engine temp. above 125°F, etc.), stored fuel vapors are purged from the canister whenever the purge VSV is opened by the ECM. At the appropriate time, the ECM will turn on the purge VSV.
The ECM will change the duty ratio cycle of the purge VSV thus controlling purge flow volume. Purge flow volume is determined by manifold pressure and the duty ratio cycle of the purge VSV. Atmospheric pressure is allowed into the canister to ensure that purge flow is constantly maintained whenever purge vacuum is applied to the canister (see Figure 1).
ORVR Operation
During refueling, low pressure above the diaphragm in the on-board recovery valve lifts allowing fuel vapors into the charcoal canister. At the same time, the air drain valve opens and the charcoal absorbs the fuel vapors (see Figure 2).
Early Type (Non-Intrusive) EVAP System DTCS
EVAP Monitor Leak Operation P0440
The ECM tests for leaks by measuring EVAP system pressure in the lines, charcoal canister, and fuel tank. When the EVAP pressure is higher or lower than atmospheric pressure, the ECM concludes that no leaks are present. EVAP pressure is measured by the vapor pressure sensor. If either the tank or canister purge side is at atmospheric pressure under specific conditions, the ECM determines there is a leak.
If DTC P0440 is present, the leak is on the fuel tank side of the EVAP system. This also includes the lines between the fuel tank and part of the canister. When the Vapor Pressure sensor is measuring tank pressure, the ECM is observing changes in pressure and comparing tank pressure to atmospheric pressure. No difference in pressure indicates a leak. The ECM may take 20 minutes or more to complete testing the fuel tank side (see Figure 3).
Canister Leak Detection P0446
When the ECM switches the vapor pressure VSV to canister side, the ECM measures canister pressure. A leak on the canister side can set multiple DTCs (see Figure 4).
Vapor Purge Flow P0441
The EVAP monitor is designed to detect:
^ Restricted vapor purge flow when the purge VSV is open
^ Inappropriate vapor purge flow when the purge VSV is closed
Under normal purge conditions, pressure pulsations generated by the cycling of the purge VSV are present in the canister and detected by the Vapor Pressure sensor.
Three-Way VSV P0446
The three-way VSV is connected to the Vapor Pressure sensor, canister, and fuel tank.
This VSV allows the Vapor Pressure sensor to detect either canister or tank pressure.
There are two modes the ECM can use to determine if the three-way VSV is malfunctioning. The three-way VSV is judged to be normal if there is pressure difference between the tank and canister when the three-way VSV is switched to look at the charcoal canister and fuel tank side of system.
If there isn't any pressure difference between the fuel tank and canister, the ECM looks for the following conditions:
^ During purging, pressure pulsations generated by the purge VSV are not present in the canister as detected by Vapor Pressure sensor, the three-way VSV is judged to be defective.
^ If there are pressure pulsations detected by the Vapor Pressure sensor present in the fuel tank, the three-way VSV is judged to be defective.