DESCRIPTION
Carbon-canister storage is the system used for fuel evaporation control on all vehicles. This reduces the amount of gasoline vapor emissions (hydrocarbons).
Fuel vapors are stored in the carbon canister until they can be drawn into the engine for burning during the combustion process. This system burns fuel vapors in the engine rather than venting vapors into the atmosphere.
The fuel tank and carburetor fuel bowl are vented through a hose into a canister containing activated carbon. The carbon adsorbs the fuel vapors when the engine is not operating. When the engine is started, fuel vapors are drawn from the canister into the engine. (Scheme 42)and (Scheme 45).
Carbon-canisters are either open or closed-bottom in design. Open-bottom canisters draw outside air into system through a filter in bottom of the canister. Some models use an electric solenoid or a thermostatically controlled vacuum valve, combined with ported vacuum, to control the canister purge function. (Scheme 43)
Scheme 42
Scheme 43
AUXILIARY CANISTER
An auxiliary canister is added to primary canister to increase canister capacity. Auxiliary canister is connected in line to purge air inlet of primary canister.
Vapors overflowing from float bowl are stored in auxiliary canister. During purging, excess vapors are routed to intake manifold for burning in engine.
CANISTER CONTROL VALVE
There are 2 types of canister control valves. The first type of valve performs same function as fuel bowl vent and canister purge. When engine is not running, spring tension holds valve open allowing venting of float bowl. When engine is running, manifold vacuum pulls plunger up to close valve.
The second type of valve performs function of both vapor vent valve and purge valve. When engine is running, manifold vacuum from PCV system pulls lower diaphragm upward. This turns off venting of carburetor float bowl.
When engine is running above idle speed, control vacuum pulls upper diaphragm upward. This allows purging of canister through PCV system. (Scheme 44)
Scheme 44
2-TUBE CANISTER
Gasoline vapors from fuel tank flow into canister and are adsorbed by carbon. Canister is purged when engine is running above idle speed.
A timed vacuum source is applied to canister to draw fresh air through bottom of canister. Fresh air mixes with fuel vapors and are drawn into intake manifold to be burned in engine.
Some models are equipped with a second canister purge hose. connected to carburetor bowl vent line.
This allows carburetor bowl to be vented to canister, through CCV, when engine is not running. These vapors are then drawn into intake manifold once engine is started.
FUEL TANK PRESSURE CONTROL VALVE
Fuel tank pressure control valve is a spring controlled valve. When engine is running, vacuum is applied to valve and valve is opened. This allows vapors from fuel tank to vent to canister.
When engine is not running, valve closes. This causes fuel tank vapors to be vented through restriction in valve. This restriction is to retain most of fuel tank vapors in fuel tank.
PURGE SOLENOID VALVE
Purge solenoid valve is controlled by Electronic Control Module (ECM). When energized, purge solenoid valve turns off causing vapors to be drawn from canister through restrictions. When de-energized, valve opens, by-passing restriction and allowing high purge.
For high purge to occur, following conditions must be met: fuel control system must be operating in closed loop, time of 175 seconds must pass after starting engine, and vehicle must be above 20 MPH (automatic transmission) or above 1300 RPM (manual transmission).
Scheme 45
FUEL BOWL VENT SOLENOID
Fuel bowl vent solenoid is a grounded solenoid that is energized by ignition voltage. When solenoid is energized, valve is turned off and opens when de-energized.
THERMAL BOWL VENT VALVE
Thermal bowl vent valve, located in a section of carburetor-to-canister hose, permits fuel bowl vapors to be directed to carbon-canister. When engine temperature is below 90°F (32°C), valve will close and will open when engine temperature is above 120°F (49°C).
THERMOSTATIC VACUUM SWITCH
Thermostatic vacuum switch is mounted on the intake manifold to sense engine coolant temperature. This 2-port, temperature operated vacuum switch, opens when temperature reaches 170°F (70°C) to allow fuel bowl vapors to escape to carbon-canister.
MAINTENANCE
Check all fuel and vapor lines for proper connections and correct routing. Remove canister and check for cracks or other damage. Replace damaged or deteriorated parts as necessary. Replace filter in bottom of canister if dirty or clogged.
CARBON-CANISTER CHECK
- Install a short length of hose on carburetor bowl vent tube of canister. Blow into hose. If air will not pass through hose, canister must be replaced.
- Using a hand vacuum pump, apply 15 in. Hg to vacuum signal tube on diaphragm assembly cover. Diaphragm should hold vacuum for at least 20 seconds. If diaphragm does not hold vacuum, diaphragm is leaking and canister must be replaced.
- With vacuum still applied to diaphragm, try to blow through carburetor bowl vent hose on canister. If air does not enter canister past vapor vent valve, vent valve is not functioning properly. Replace canister.
PURGE SOLENOID VALVE CHECK
For testing of Purge Solenoid Valve, see appropriate diagnostic chart in COMPUTER COMMAND CONTROL article.