AMC Coupes/Sedans/Wagons 1975-1988 Repair Information

Fuel Tank Vapor Emission Control System



A closed fuel tank system is used on some 1975-78, and all 1979 and later models, to route raw fuel vapor from the fuel tank into the PCV system (sixes) or air cleaner snorkle (V8s), where it is burned along with the fuel/air mixture. The system prevents raw fuel vapors from entering the atmosphere.

The fuel vapor system consists of internal fuel tank venting, a vacuum-pressure fuel tank filler cap, an expansion tank or charcoal filled canister, liquid limit fill valve, and internal carburetor venting.

Fuel vapor pressure in the fuel tank forces the vapor through vent lines to the expansion tank or charcoal filled storage canister. The vapor then travels through a single vent line to the limit fill valve, which regulates the vapor flow to the valve cover or air cleaner.

Limit Fill Valve

This valve is essentially a combination vapor flow regulator and pressure relief valve. It regulates vapor flow from the fuel tank vent line into the valve cover. The valve consists of a housing, a spring loaded diaphragm and a diaphragm cover. As tank vent pressure increases, the diaphragm lifts, permitting vapor to flow through. The pressure at which this occurs is 4-6 in. H 2 O column. This action regulates the flow of vapors under severe conditions, but generally prohibits the flow of vapor during normal temperature operation, thus minimizing driveability problems.

Liquid Check Valve

The liquid check valve prevents liquid fuel from entering the vapor lines leading to the storage canister. The check valve incorporates a float and needle valve assembly. If liquid fuel should enter the check valve, the float will rise and force the needle upward to close the vent passage. With no liquid fuel present in the check valve, fuel vapors pass freely from the tank, through the check valve, and on to the storage canister.


There are several things to check for it a malfunction of the evaporative emission control system is suspected.

  1. Leaks may be traced by using an infrared 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.
  3. Leaks may be caused by any of the following, so always check these areas when looking for them:
    1. Defective or worn lines.
    3. Disconnected or pinched lines.
    5. Improperly routed lines.
    7. A defective filler cap.


If it becomes necessary to replace any of the lines used in the evaporative emission control system, use only hoses which are fuel resistant or are marked `EVAP'.

  1. 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.
  3. 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.

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Location of the Evaporative Emission Control canister