Gasoline fuel is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated, from the refinery standpoint. However, the automobile still remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.
Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in warm area, such as a parking lot. As the fuel would warm, it would expand and should no provisions or area be provided for the expansion, the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition, the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.
However, this did not control the fuel vapor emission from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was stationary and the engine not operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor emissions would take place, increasing as the temperature increases.
To prevent the vapor emission from escaping into the atmosphere, the fuel systems were designed to trap the vapors while the vehicle is stationary, by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is not operating. When the engine is started, the storage system is then purged of the fuel vapors, which are drawn into the engine and burned with the air/fuel mixture.
The basic Evaporative Emission (EVAP) Control system used on these vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by the intake air flow and consumed in the normal combustion process.
The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). Poor idle, stalling and poor driveability can be caused by: a malfunctioning purge solenoid, a damaged canister; hoses and lines split, cracked or not connected properly (perhaps after rear end collision body work to the vehicle).
The Evaporative Emission Control (EVAP) System is designed to prevent fuel tank vapors from being emitted into the atmosphere. Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air which escapes contains gasoline vapors.
The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through the pressure control valve, through vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating, and at normal operating temperature, the vapors are drawn into the intake manifold and burned during combustion..
A sealed, maintenance free evaporative canister is used. The canister is filled with granules of an activated carbon mixture. Fuel vapors entering the canister are absorbed by the charcoal granules. A vent cap is located on the top of the canister to provide fresh air to the canister when it is being purged. The vent cap opens to provide fresh air into the canister, which circulates through the charcoal, releasing trapped vapors and carrying them to the engine to be burned.
Fuel tank pressure vents fuel vapors into the canister. They are held in the canister until they can be drawn into the intake manifold. The canister purge valve allows the canister to be purged at a pre-determined time and engine operating conditions.
The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valve in the cap is a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.
The fuel tank pressure sensor is a three-wire strain gauge sensor much like that of the common GM MAP sensor. However, this sensor has very different electrical characteristics due to the pressure differential design. The sensor measures the difference between the air pressure (or vacuum) in the fuel tank and the outside air pressure. The sensor mounts at the top of the fuel tank. A three-wire electrical harness connects it to the Powertrain Control Module (PCM). The PCM supplies a five volt reference voltage and ground to the sensor. The sensor will return a voltage between 0.1 and 4.9 volts. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will be 1.3 to 1.7 volts. This sensor is the reason the fuel filler door is usually placarded to remind motorists to tighten the fuel filler cap after refueling. If the tank pressure sensor doesn't see a difference between the air pressure (or vacuum) in the fuel tank and the outside air pressure, the PCM will set a Diagnostic Trouble Code (DTC) and may light up the Malfunction Indicator Lamp (MIL) on the instrument panel.
Enhanced emission testing requires that vehicles be equipped with an EVAP test port. This is located in the evaporative purge hose, located between the purge solenoid and the canister. The service port is identified by a green color cap. The port contains a schrader valve and fittings which allow connections to an EVAP Diagnostic Station, now required in many emission tests.
Poor engine idle, stalling and poor driveability can be caused by an inoperative canister purge solenoid, a damaged canister or split, damaged or improperly connected hoses.
The most common symptom of problems in this system is fuel odors coming from under the hood. If there is no liquid fuel leak, check for a cracked or damaged vapor canister, inoperative or always open canister control valve, disconnected, misrouted, kinked or damaged vapor pipe or canister hoses; or a damaged air cleaner or improperly seated air cleaner gasket.
Removal & Installation
Evaporative Emission Canister
- Raise and safely support the vehicle.
- Remove the left (driver's side) rear tire assembly.
- Remove the left rear wheel hose liner (inner fender).
- Tag for identification the multiple hoses and disconnect the hoses from the canister.
- Loosen the latch and/or retaining screw(s) on the canister bracket.
- Remove the canister from the vehicle.
- Installation is the reverse of the removal procedure, taking care to connect the hoses properly, referencing the identification tags made at removal. If necessary, refer to the Vehicle Emission Control Information (VECI) label, located in the engine compartment, for proper routing of the vacuum hoses.
The EVAP and Enhanced EVAP system monitor is incorporated in the OBD-II engine control system. A pressure sensor is mounted on the fuel tank which measures pressure inside the tank, and a purge flow sensor measures the flow of the gases from the canister into the engine. The vent valve for the canister is now controlled by the PCM. It performs the same functions as the purge valve, however it looks slightly different. A canister vent solenoid is mounted on the canister, taking the place of the vent cap, providing a source of fresh air to the canister.
The PCM can store trouble codes for EVAP system performance, a list of the codes is provided later in this section.
As part of the OBD-II diagnostic system, a test port is installed in the purge line connecting the purge solenoid to the canister. This test port is used to connect a special tester that pressurizes the EVAP system and measure the leakage rate. This tool is very expensive and requires training to operate or you could do damage to the vehicle, however, many professional shops have this device. If you suspect a leak in your EVAP system, you may want to consult a professional shop about performing this test on your car.
A visual inspection is one of the best checks of a EVAP or Enhanced EVAP system operation.
Check for the following conditions:
- Poor connection at the PCM. Inspect the PCM harness connectors for backed out terminals, improper or loose connections, broken clips or locks, damaged or corroded terminals, and poor terminal to wire connection.
- Inspect the wiring harness for damage.
- Inspect for incorrect purge or vacuum source line routing.
- Visually check the canister for cracks or damage.
- If fuel is leaking from the bottom of the canister, replace canister and check for proper hose routing.
- If so equipped, check the filter at the bottom of the canister. If dirty, replace the filter.
The evaporative canister is not a serviceable unit. If found to be defective, replace the entire unit. On all of these vehicles, the EVAP canister is located in the wheel well behind the driver's side rear wheel. The canister and valve can be serviced only by replacement. For further information, please see Section 4.