Ford Pick-Ups/Bronco 1976-1986 Repair Guide

General Information


See Figures 1, 2, 3 and 4

Changes in atmospheric temperature cause fuel tanks to breathe; that is, the air within the tank expands and contracts with outside temperature changes. As the temperature rises, air escapes through the tank vent tube or the vent in the tank cap. The air which escapes contains gasoline vapors. In a similar manner on carbureted engines, the gasoline which fills the carburetor float bowl expands when the engine is stopped. Engine heat causes this expansion. The vapors escape through the air cleaner.

The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. The system has three parts: a fill control vent system; a vapor vent and storage system; and a pressure and vacuum relief system (special fill cap).

The fill control vent system is a modification to the fuel tank. It uses a dome air space within the tank which is 10-12% of the tank's volume. The air space is sufficient to provide for the thermal expansion of the fuel. The space also serves as part of the in-tank vapor vent system.

The in-tank vent system consists of the domed air space previously described and a vapor separator assembly. The separator assembly is mounted to the top of the fuel tank and is secured by a cam-lockring, similar to the one which secures the fuel sending unit. Foam material fills the vapor separator assembly. The foam material separates raw fuel and vapors, thus retarding the entrance of fuel into the vapor line.

The vapor separator is an orifice valve located in the dome of the tank. The restricted size of the orifice, 0.050 in. (1.27mm) tends to allow only vapor to pass out of the tank. The orifice valve is connected to the vent line which runs forward to the carbon filled canister in the engine compartment.

The sealed filler cap has a pressure-vacuum relief valve. Under normal operating conditions, the filler cap operates as a check valve, allowing air to enter the tank to replace the fuel consumed. At the same time, it prevents vapors from escaping through the cap. In case of excessive pressure within the tank, the filler cap valve opens to relieve the pressure.

Because the filler cap is sealed, fuel vapors have only one place through which they may escape: the vapor separator assembly at the top of the fuel tank. The vapors pass through the foam material and continue through a single vapor line which leads to a canister in the engine compartment. The canister is filled with activated charcoal.

Another vapor line runs from the top of the carburetor float chamber or the intake manifold, or the throttle body, to the charcoal canister.

As the fuel vapors (hydrocarbons), enter the charcoal canister, they are absorbed by the charcoal. The air is dispelled through the open bottom of the charcoal canister, leaving the hydrocarbons trapped within the charcoal. When the engine is started, vacuum causes fresh air to be drawn into the canister from its open bottom. The fresh air passes through the charcoal picking up the hydrocarbons which are trapped there and feeding them into the engine for burning with the fuel mixture.

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Fig. Fig. 1: Common canister venting system-1983 V-6 engine shown

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Fig. Fig. 2: Fuel lines and evaporative emission control devices-Aft steel tank-F-100, F250 Regular Cab

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Fig. Fig. 3: Common evaporative emission system components-4.9L F-150-F-350 and Bronco

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Fig. Fig. 4: Evaporative emission system components-California V-8 engine