The basic evaporative emission control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake airflow and consumed in the normal combustion process.
TESTING AND CHECKING
Before embarking on component removal or extensive diagnosis, perform a complete visual check of the system. Every vacuum line and vapor line (including the lines running to the tank) should be inspected for cracking, loose clamps, kinks and obstructions. Additionally, check the tank for any signs of deformation or crushing. Each vacuum port on the engine or manifold should be checked for restriction by dirt or sludge.
The evaporative control system is generally not prone to component failure in normal circumstances; most problems can be tracked to the causes listed above.Fuel Filler Cap
Check that the filler cap seals effectively. Remove the filler cap and pull the safety valve outward to check for smooth operation. Replace the filler cap if the seal is defective or if it is not operating properly.Charcoal Canister
See Figures 1 and 2
- Label and disconnect the lines running to the canister. Remove the charcoal canister from the vehicle.
- Visually check the charcoal canister for cracks or damage.
- Check for a clogged filter and stuck check valve. Using low pressure compressed air, blow into the tank pipe and check that the air flows without resistance from the other pipes. If this does not test positive replace the canister.
- Clean the filter in the canister by blowing no more than 43 psi of compressed air into the pipe to the outer vent control valve while holding the other upper canister pipes closed.
Do not attempt to wash the charcoal canister. Also be sure that no activated carbon comes out of the canister during the cleaning process.
- Replace or reinstall the canister as needed.
NOVA WITH VIN CODE 4 ENGINE
See Figure 3
- Label and disconnect the hoses from the control valve but leave the wiring for the valve connected.
- Check that the valve is open by blowing air through it when the ignition switch is in the OFF position.
- Check that the valve is closed when the ignition switch is in the ON position.
- Reconnect the hoses to the proper locations. If the valve doesn't operate correctly, double check the fuse and wiring before replacing the valve.
NOVA WITH VIN CODE 4 ENGINE
See Figure 4
- Drain the coolant from the radiator into a clean container.
- Remove the thermoswitch from the intake manifold. The switch is located behind the TVSV (Thermo Vacuum Switching Valve).
- Cool the thermo switch off until the temperature is below 109°F (43°C). Check that there is continuity through the switch by the use of an ohmmeter.
- Using hot water, bring the temperature of the switch to above 131°F (55°C). Check that there is no continuity when the switch is in water above this temperature.
- Apply sealer to the threads of the switch and reinstall it in the manifold.
- Refill the radiator with coolant.
REMOVAL & INSTALLATION
Removal and installation of the various evaporative emission control system components consists of labeling and unfastening hoses, loosening retaining screws, and removing the part which is to be replaced from its mounting point.
When replacing any EVAP system hoses, always use hoses that are fuel-resistant or are marked EVAP. Use of hose which is not fuel-resistant will lead to premature hose failure.