Ford Mustang/Capri 1979-1988 Repair Guide

Fuel Evaporative Control System


See Figures 1 and 2

This system is designed to prevent the evaporation of unburned gasoline. The system consists of a vacuum/pressure relief fuel filler cap, an expansion area at the top of the fuel tank, a foam-filled vapor separator mounted on top of the fuel tank, a carbon canister which stores fuel vapors and a number of hoses which connect the various components. The system functions as follows:

Changes in ambient temperature cause the gasoline in fuel tanks to expand or contract. If this expansion and consequent vaporization takes place in a conventional fuel tank, the vapors escape through the filler cap or vent hose and pollute the atmosphere. The fuel evaporation emission control system prevents this by routing the gasoline vapors to the engine where they are burned.

As the gasoline in the fuel tank of a parked car begins to expand due to heat, the vapor that forms moves to the top of the fuel tank. The tanks are enlarged so that there is an area representing 10-20% of the total fuel tank volume above the level of the fuel tank filler tube where these gases may collect. The vapors travel upward into the vapor separator, which prevents liquid gasoline from escaping the fuel tank. The vapor then travels through the separator outlet hose to the charcoal canister in the engine compartment. The vapor enters the canister, passes through a charcoal filter, and then exits through the canister's atmospheric outlet. As the vapor passes through the charcoal, hydrocarbons are removed, so pollutants passing through to the atmosphere are reduced.

When the engine is started, intake vacuum draws fresh air into the canister. As the entering air passes through the charcoal in the canister, it picks up the hydrocarbons that were deposited by the fuel vapors. This mixture of hydrocarbons and fresh air is then carried through a hose to the carburetor or air cleaner assembly. It combines with the incoming air/fuel mixture in the carburetor or throttle body, and enters the combustion chambers of the engine where it is burned.

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Fig. Fig. 1: Evaporative emissions control canister

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Fig. Fig. 2: Vapor separator


The only required service for the evaporative emissions control system is inspection of the various components at the interval specified in the maintenance chart (see General Information & Maintenance ). If the charcoal element in the canister becomes gummed up, the entire canister should be replaced. Disconnect the canister purge hose from the air cleaner fitting, loosen the canister retaining bracket and lift out the canister. Installation is the reverse of removal.