All vehicles have an Evaporation Control System to reduce evaporation losses from the fuel system. The system has an expansion tank in the main fuel tank. This prevents spillage due to expansion of warm fuel. A special filler cap with a two-way relief valve is used. An internal pressure differential, caused by thermal expansion, opens the valve, as does an external pressure differential caused by fuel usage. Fuel vapors from the carburetor and fuel tank are routed to the crankcase ventilation system. A separator is installed to prevent liquid fuel from entering the crankcase ventilation system.
Evaporation control systems also include a charcoal canister and an overflow limiting valve.
The limiting valve prevents the fuel tank from being overfilled by trapping fuel in the filler when the tank is full. When pressure in the tank becomes greater than the valve operating pressure, the valve opens and allows the gasoline vapors to flow into the charcoal canister.
The charcoal canister is mounted in the engine compartment. It absorbs vapors and retains them until clean air is drawn through a line from it that runs to the PCV valve. Absorption occurs while the car is parked and cleaning occurs when the car engine is running.
Some models are equipped with duel canisters.
There are several things to check for if a malfunction of the evaporative emission control system is suspected.
- Leaks may be traced by using an infra-red hydrocarbon tester. Run the test probe along the lines and connections. The meter will indicate the presence of a leak by a high hydrocarbon (HC) reading. This method is much more accurate than a visual inspection which would indicate only the presence of a leak large enough to pass liquid.
- Leaks may be caused by any of the following, so always check these areas when looking for them:
- Defective or worn lines;
- Disconnected or pinched lines;
- Improperly routed lines;
- A defective filler cap.
If it becomes necessary to replace any of the lines used in the evaporative emission control system, use only those hoses which are fuel resistant or are marked "EVAP."
- If the fuel tank has collapsed, it may be the fault of clogged or pinched vent lines, a defective vapor separator, or a plugged or incorrect fuel filler cap.
- To test the filler cap, clean it and place it against the mouth. Blow into the relief valve housing if the cap passes pressure with light blowing or if it fails to release with hard blowing, it is defective and must be replaced.
Replace the cap with one marked "pressure/vacuum" only. An incorrect cap will render the system inoperative or damage its components.