When raw fuel evaporates, the vapors contain hydrocarbons. To prevent these fumes from escaping into the atmosphere, the fuel evaporative emission control system was developed.
There are two different evaporative emission control systems used on Datsun/Nissan. The system used through 1974 consists of a sealed fuel tank, a vapor/liquid separator, a flow guide (check) valve, and all of the hoses connecting these components, in the above order, leading from the fuel tank to the PCV hose, which connects the crankcase to the PCV valve.
In operation, the vapor formed in the fuel tank passes through the vapor separator, onto the flow guide valve and the crankcase. When the engine is not running, if the fuel vapor pressure in the vapor separator goes about 0.4 in. Hg, the flow guide valve opens and allows the vapor to enter the engine crankcase. Otherwise the flow guide valve is closed to the vapor separator while the engine is not running. When the engine is running, and a vacuum is developed in the fuel tank or in the engine crankcase and the difference of pressure between the relief side and the fuel tank or crankcase becomes 2 in. Hg, the relief valve opens and allows ambient air from the air cleaner into the fuel tank or the engine crankcase. This ambient air replaces the vapor within the fuel tank or crankcase, bring the fuel tank or crankcase back into a neutral or positive pressure range.
The system used on 1975 and later models consists of a sealed fuel tank, a vapor/liquid separator (certain models only), a vapor vent line, a carbon canister, a vacuum signal line and a canister purge line.
In operation, fuel vapors and/or liquid are routed to the liquid/vapor separator or check valve where liquid fuel is directed back into the fuel tank as fuel vapors flow into the charcoal filled canister. The charcoal absorbs and stores the fuel vapors when the engine is not running or is at idle. When the throttle valves in the carburetor (or air intakes for fuel injection) are opened, vacuum from above the throttle valves is routed through a vacuum signal line to the purge control valve on the canister. The control valve opens and allows the fuel vapors to be drawn from the canister through a purge line and into the intake manifold and the combustion chambers.
INSPECTION AND SERVICE
Check the hoses for proper connections and damage. Replace as necessary. Check the vapor separator tank for fuel leaks, distortion and dents, and replace as necessary.Flow Guide Valve
1973-74 MODELS ONLY
Remove the flow guide valve and inspect it for leakage by blowing air into the ports in the valve. When air is applied from the fuel tank side, the flow guide valve is normal if the air passes into the check side (crankcase side), but not into the relief side (air cleaner side). When air is applied from the check side, the valve is normal if the passage of air is restricted. When air is applied from the relief side (air cleaner side), the valve is normal if air passes into the fuel tank side or into the check side.Carbon Canister and Purge Control Valve
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 maintained 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. The filter element should be checked once a year or every 12,000 miles; more frequently if the car is operated in dusty areas. Replace the filter by pulling it out of the bottom of the canister and installing a new one.
REMOVAL & INSTALLATION
Removal and installation of the various evaporative emission control system components consists of disconnecting the hoses, loosening retaining screws, and remove the part which is to be replaced or checked. Install in the reverse order. When replacing hose, make sure that it is fuel and vapor resistant.