Evaporative Emissions System: Description and Operation

This system is used to prevent fuel vapors emitted from the fuel tank and carburetor from escaping into the atmosphere. This is accomplished through the use of a non-vented fuel tank, a special fuel filler cap, charcoal canister (some applications), vapor separators, check valves, vented carburetor, and hoses necessary to connect these components together.
Fuel vapors are absorbed and stored in the charcoal canister until the engine is started. These vapors are then drawn either through the air cleaner snorkel or through a ported vacuum port in the carburetor (depending on application) and burned along with the normal air/fuel mixture.

FUEL TANK
The fuel tank is equipped with an integral expansion area directly above the fuel to allow for normal fuel expansion. Some vehicles with this type of fuel tank are equipped with a liquid check valve to prevent fuel from entering the vapor lines.

FUEL FILLER CAP
The fuel filler cap uses a two-way (vacuum and pressure) relief valve that is closed during normal operating conditions. The relief valve is calibrated to open only when pressure in excess of .8 psi or vacuum in excess of .1 inch Hg develops in the fuel tank. When vacuum or pressure is relieved, the valve returns to its normally closed position.
Some filler caps have a rollover check valve. This valve consists of a stainless steel ball in a plastic housing mounted on the fuel tank side of the cap. Should the vehicle be overturned or tipped sufficiently, the steel ball drops into an orifice closing the vent to prevent fuel leakage. It is normal to occasionally encounter an air pressure release when removing the fuel filler cap.

FUEL VAPOR STORAGE CANISTER (CHARCOAL CANISTER)
The fuel resistant nylon body of the canister contains activated charcoal granules which absorb and store fuel vapors until they are drawn into the induction system to be burned along with the normal air fuel mixture. Outside air is drawn into the canister through a replaceable filter pad located at the bottom of the canister.
The canister has a staged dual feature and four nipples. Two of the nipples are inlets, one for fuel tank vapor and the other for carburetor float bowl vapor. The other two nipples are outlets, one is connected to intake manifold vacuum while the other is for carburetor ported vacuum. When the engine is running, manifold vacuum causes fresh air to enter through the inlet filter in the canister and purge the stored vapor. When ported vacuum increases due to increased throttle opening, the secondary purge circuit is opened and the canister is purged at a much higher rate.

Fig. 13 Liquid check valve. Typical:

LIQUID CHECK VALVE
This valve, Fig. 13, contains a float and Viton needle assembly. Should liquid fuel enter the check valve, the float will rise and force the needle upward closing the vent passage and preventing fuel from flowing through the valve. Two different design valves are used, one picks up vapors from the lines while the other picks up vapors from the fuel tank. Both operate the same.
After passing through the check valve, the fuel vapor is routed forward through a vent line to the vapor storage canister in the engine compartment.

Fig. 14 Rollover check valve. CJ models:

Fig. 15 Rollover check valve. All except CJ models:

ROLLOVER CHECK VALVE
This valve, Figs. 14 and 15, prevents the flow of fuel through the fuel line should the vehicle be rolled over. The check valve consists of a plunger and a stainless steel ball. When inverted, the stainless steel ball pushes the plunger against its seat.

Fig. 16 Carburetor external bowl vent:

CARBURETOR EXTERNAL BOWL VENT
This vent, Fig. 16, used on some carburetors provides an outlet from fuel vapors when the engine is not running. If this vent were not provided, raw fuel vapors would enter the atmosphere. Some vapors would also enter the intake manifold making hot restarts difficult. When the engine is running, the fuel bowl must be vented to the inside of the air cleaner for proper fuel flow. This is accomplished by automatically closing the bowl vent. Some carburetors use manifold vacuum to accomplish this while others use a mechanical link to the throttle.




FUEL RETURN
Most vehicles use a fuel return system to reduce the possibility of high temperature fuel problems. This system consists of a line connecting an extra nipple on the fuel filter to an extra nipple on the fuel tank sending unit. During normal engine operation, a small amount of fuel returns to the fuel tank. During periods of high underhood temperature, vaporized fuel is returned to the fuel tank instead of entering the carburetor.
Some vehicles have a check valve in the fuel return system at the fuel filter to eliminate the possibility of fuel feeding back to the carburetor through the fuel return line. This valve uses a stainless steel, spring loaded check ball to close the orifice. It is located on the fuel return line and secured by a clamp.