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Evap system/gas cap message

Cadillac Tech

Seasoned Member
Oct 16, 2008
I thought this might help some of you who have been having "check gas cap" message and or a check engine light.

XLR (VIN Y) Service Manual | Engine | Engine Controls - 4.6L (LH2) | Description and Operation | Document ID: 1187819
Evaporative Emission Control System Description

EVAP System Operation

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors 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 solenoid 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 solenoid ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, fresh air is drawn through the vent solenoid and the 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 solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test

This tests for large leaks and blockages in the EVAP system. The control module commands the EVAP vent solenoid ON and commands the EVAP purge solenoid ON, with the engine running, allowing engine vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage 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 solenoid OFF, 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 rate is more than a calibrated level on 2 consecutive tests, DTC P0455 will set.

Small Leak Test

The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the EVAP system. The EONV diagnostic monitors the EVAP system pressure or vacuum with the key OFF. Because of this, it may be normal for the control module 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 EONV.
The EONV utilizes the temperature changes and the resulting naturally occurring vacuum or pressure in the fuel tank immediately following a drive cycle. 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, then begins to decrease. The EONV diagnostic relies on this temperature change and it's corresponding pressure change to determine if an EVAP system leak is present.
The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 inches). The diagnostic 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, little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.

Canister Vent Restriction Test

If the EVAP vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP purge solenoid ON, commanding the EVAP vent solenoid OFF, and monitoring the FTP sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will 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 control module tests for this by commanding the EVAP purge solenoid OFF and the vent solenoid ON, sealing the system, and monitors the FTP for an increase in vacuum. If the control module detects that the EVAP system vacuum increases above a calibrated value, DTC P0496 will set.

Check Gas Cap Message

The PCM sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when any of the following occur:
• A malfunction in the EVAP system and a large leak test fails
• A malfunction in the EVAP system and a small leak test fails​

EVAP System Components

The EVAP system consists of the following components:
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.

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

EVAP Vent Solenoid

The EVAP vent solenoid controls fresh airflow into the EVAP canister. The solenoid is normally open. The control module commands the solenoid ON, closing the solenoid during some EVAP tests, allowing the system to be tested for leaks.

Fuel Tank Pressure Sensor

The 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 to the control module that can vary between 0.1-4.9 volts. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.

EVAP Service Port

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

© 2008 General Motors Corporation. All rights reserved.

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