Mazda Trucks 1972-1986 Repair Guide

Evaporative Emission Control System


The evaporative emission control system is designed to control the emission of gasoline vapors into the atmosphere. The system consists of a fuel tank, a condenser tank and a check valve. In 1977, the system was changed slightly to consist of a sealed fuel tank, a vapor controlling orifice in the line between the tank and the charcoal canister, and the canister. The check valve of the previous system was eliminated through the substitution of a filler cap with vacuum and pressure relief valves, and a fuel vapor valve on the tank.

When the engine is not running, fuel vapors are channeled to the condenser tank. The fuel returns to the fuel tank as the vapors condense. During periods of engine operation, fuel vapor that has not condensed in the condenser tank moves to the carbon canister. The stored vapors are removed from the charcoal by fresh air moving through the inlet hole in the bottom of the canister.

The check valve (1972-76), located between the condenser tank and the canister, allows fuel vapor and ventilation to flow during normal operation. If the system becomes clogged or frozen, the valve opens (by negative pressure in the fuel tank) to allow fuel to be drawn from the tank. The valve also opens to vent internal tank pressure under hot conditions. The fuel tank cap and fuel vapor valve replace the check valve in 1977 and later, and perform essentially the same functions.

The only service necessary for this system is replacement of the charcoal canister at regular intervals and inspection of the rubber hoses at the same time for cracks or breaks. Any deteriorated hoses should be replaced.


  1. Raise and support the rear of the truck.
  3. Disconnect the hoses from the condenser tank.
  5. Unbolt and remove the condenser tank.
  7. Install the tank and tighten the bolts.
  9. Connect the hoses to the tank.
  11. Lower the truck.

Check Valve

  1. Disconnect the hoses from the check valve.
  3. Unscrew and remove the valve.
  5. Position a new valve on the crossmember. Tighten the attaching screws.
  7. Connect all hoses to the valve.