Evaporative Emissions System: Description and Operation

CAUTION: Exercise care when using a torch near the fuel evaporation system to prevent fire or explosion.

NOTE: When replacing vapor hoses use hoses marked "EVAP" only. Ordinary fuel hoses may deteriorate and/or clog system.

Fig. 34 - Evaporative Emission Control System. Typical:

OPERATION
In this closed system, fuel vapor is drawn from the tank--Fig 34--through a separator and lines, to a carbon-filled canister in the engine compartment.
- The liquid vapor separator passes fuel vapor to the canister and routs liquid fuel back into the tank--preventing fluid fuel from entering the EVAP system.

The fuel vapor is stored in the canister until the engine is started. Once the engine is started fuel vapors are constantly purged from the canister through a calibrated orifice in the center canister connection to the engine--via the PCV hose and/or air cleaner snorkel.

To prevent damage to the tank from excessive internal or external pressure--due to the closed system:
- A fill limiting baffle in the fuel tank reserves space for normal fuel expansion.
- The vehicle is equipped with either a relief valve in the vapor line or a filler cap designed to relieve abnormally high pressure or vacuum within the tank.

CAUTION: Be sure to use the correct filler cap for the application.
- Installation of a a non-emission vented fill cap on a sealed system tank will render the system inoperative.
- Installation of a non-vented fill cap on a conventional fuel tank will result in serious deformation or total collapse of the fuel tank.

SCHEDULED MAINTENANCE
Replace filter--mounted at the bottom of the canister:
- With light duty emissions-every 30,000 miles
- With heavy duty emissions-every 2 years, or 24,000 miles

NOTE: The purge valve--on some applications--can be repaired using repair kit, part No. 7041344. The service is NOT a routine maintenance item and should be performed only if valve is damaged or parts are missing to avoid replacing the complete vapor canister,.