Isuzu Cars and Trucks 1981-1991

Evaporative Emission Controls

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OPERATION

To limit gasoline vapor discharge into the air this system is designed to trap fuel vapors, which normally escape from the fuel tank and carburetor. Vapor arrest is accomplished through the use of the charcoal canister. This canister absorbs fuel vapors and stores them until they can be removed to be burned in the engine. Removal of the vapors from the canister to the engine is accomplished by a carburetor, throttle body assembly or solenoid operated bowl vent. In addition to the carburetor modifications and the canister, the fuel tank requires a non-vented gas cap. The domed fuel tank positions a vent high enough above the fuel to keep the vent pipe in the vapor at all times. The single vent pipe is routed directly to the canister. From the canister, the vapors are routed to the PCV system, where they will be burned during normal combustion.



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Fig. Evaporative emission control G180Z and G200Z with carburetor



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Fig. Evaporative emission control 4ZD1 carbureted engine



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Fig. Evaporative emission control G200Z and 4ZC1-T EFI engine



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Fig. Evaporative emission control 4XC1-Turbo engine



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Fig. Evaporative emission control 4XC1-U carbureted engine



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Fig. Evaporative emission control 4ZE1 EFI engine



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Fig. Evaporative emission control V6 engines

SERVICING

Make sure the hoses are connected properly and not damaged.

G180Z and G200Z Carbureted: The check and relief valve controls the amount of vapor that goes into the engine. When the pressure in the tank becomes 0.20.6 in. Hg the check valve opens, allowing the vapor into the engine crankcase.



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Fig. Check and relief valve G180Z and G200Z carbureted engines



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Fig. Ventilation valve G180Z and G200Z carbureted engines

4XC1-U carbureted: When the engine is not running, fuel vapor from the tank reaches the tank pressure control valve and passes through the bypass orifice into the canister. When the engine is running, manifold vacuum forces the valve to open and vapor is stored in the canister.

The vent switching valve is an electrically operated valve built into the carburetor. When the engine is not running, the valve opens to introduce vapor into the canister. When the engine is running, the valve closes to block the flow of ambient air from the canister to the carburetor.

The thermal vacuum valve (TVV) controls the amount of vacuum to the canister purge valve.



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Fig. Tank pressure control valve 4XC1-U engine



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Fig. Vent switching valve 4XC1-U engine



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Fig. Thermal vacuum valve 4XC1-U engine



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Fig. Vacuum switching valve 4XC1-U engine

4XC1-T Fuel Injected: The vacuum switching valve and vacuum reservoir tank are connected in series on the vacuum line to keep the tank pressure control valve opening under the condition of positive common chamber boost. It prevents vapor purge during engine warm-up and cold driving conditions.

Check the vacuum reservoir tank by disconnecting the two vacuum hoses. Blow air into the hose at the A side of the tank, and confirm that the air flows to the B port. Confirm that the air does not flow to the A port when blowing in the B port. If a problem is found, replace the tank.

4ZD1 engine: The roll over valve and float valve is designed to prevent fuel leaks when the vehicle has turned over. The relief valve is intended to relieve the tank internal pressure to avoid tank damage.

All Vehicles, except V6 engines: Inspect the charcoal canister for cracks and other damage. Remove the canister from the vehicle to test. Apply about 7.5 psi positive pressure to the port marked VC. There should be no air leakage from the diaphragm. Apply about 15.0 in. Hg of vacuum to the port marked PURGE and gradually apply a negative pressure to the port marked VC. If the purge control valve begins to open at between 1.6 and 3.2 in. Hg of pressure as the negative gauge reading falls, the control valve is functioning normally. If not, the canister assembly must be replaced.



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Fig. Vacuum reservoir tank 4XC1-T engine



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Fig. Roll-over and relief valve 4ZD1 engine

FUNCTIONAL TEST V6 ONLY
Canister Purge Valve
  1. Apply a short length of hose to the lower tube of the purge valve and attempt to blow through the hose. Little or no air should pass through into the canister.
  2.  
  3. With a hand vacuum pump, apply 15 in. Hg (51 kPa) of vacuum through the control valve tube (upper tube). The diaphragm should hold vacuum for at least 20 seconds, if not the canister has to be replaced.
  4.  



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Fig. Checking the canister

Fuel Tank Pressure Control Valve
  1. Apply 15 in. Hg (51 kPa) of vacuum to the control vacuum tube. The diaphragm should hold vacuum for at least 20 seconds. If it does not, the diaphragm is leaking and the valve must be replaced.
  2.  
  3. With the vacuum applied to the control vacuum tube, apply a short hose to the valve's tank tube side and blow into the tube. The air should pass through the valve. If no air passes through the valve, the valve should be replaced.
  4.  

REMOVAL AND INSTALLATION

Label and disconnect the fuel canister hoses from the canister, remove the bracket bolt and canister.

 
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