Jeep CJ/Scrambler 1971-1986 Repair Guide

Fuel Tank Vapor Emission Control System


See Figures 1, 2, 3 and 4


A closed fuel tank system is used on some models through 1974, some models 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.

The fuel tank vent line is routed through the limit fill valve to the valve cover on the left side on the 1972 V8s. On the 1973 Sixes, it travels to the intake manifold and on the V8s it is routed to the carburetor 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.

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Fig. Fig. 1: Fuel tank and vent lines as used on 1972-73 models

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Fig. Fig. 2: Fuel tank and vapor control components used on 1974-78 models

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Fig. Fig. 3: Fuel tank and vapor control components used on 1979-81 models

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Fig. Fig. 4: Fuel tank and related components used on 1982-86 models

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.