See Figures 1, 2, 3 and 4Gasoline Engines Only
Check the evaporation control system every 15,000 miles (24,000 km). Check the fuel and vapor lines/hoses for proper connections and correct routing, as well as condition. Replace damaged or deteriorated parts as necessary. Remove and check the operation of the check valve on 1970-75 models in the following manner:
- With all the hoses disconnected from the valve, apply air pressure to the fuel tank side of the valve. The air should flow through the valve and exit the crankcase side of the valve. If the valve does not behave in the above manner, replace it.
- Apply air pressure to the crankcase side of the valve. Air should not pass to either of the other two outlets.
- When air pressure is applied to the carburetor side of the valve, the air should pass through to exit out the fuel tank and/or the crankcase side of the valve.
On 1975-88 models, the flow guide valve is replaced with a carbon filled canister which stores fuel vapors until the engine is started, the vapors are then drawn into the combustion chambers and burned.
To check the operation of the carbon canister purge control valve, disconnect the rubber hose between the canister control valve and the T-fitting (at the T-fitting). Apply vacuum to the hose leading to the control valve. The vacuum condition should be held indefinitely. If the control valve leaks, remove the top cover of the valve and check for a dislocated or cracked diaphragm. If the diaphragm is damaged, a repair kit containing a new diaphragm, retainer, and spring is available and should be installed.
The carbon canister has an air filter in the bottom of the canister. Most filters are replaceable. The filter element should be checked once a year or every 15,000 miles (24,000 km); more frequently if the truck is operated in dusty areas. Replace the filter by pulling it out of the bottom of the canister and installing a new one.