GM Prizm/Nova 1985-1993 Repair Guide

Evaporative Emission Control System (EVAP or EECS)

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OPERATION



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

  1. Label and disconnect the lines running to the canister. Remove the charcoal canister from the vehicle.
  2.  
  3. Visually check the charcoal canister for cracks or damage.
  4.  
  5. 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.
  6.  
  7. 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.
  8.  

Do not attempt to wash the charcoal canister. Also be sure that no activated carbon comes out of the canister during the cleaning process.

  1. Replace or reinstall the canister as needed.
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Example of checking a charcoal canister



Click image to see an enlarged view

Fig. Fig. 2: Example of cleaning a charcoal canister

Outer Vent Control Valve
NOVA WITH VIN CODE 4 ENGINE

See Figure 3

  1. Label and disconnect the hoses from the control valve but leave the wiring for the valve connected.
  2.  
  3. Check that the valve is open by blowing air through it when the ignition switch is in the OFF position.
  4.  



Click image to see an enlarged view

Fig. Fig. 3: Check the outer vent control valve with the ignition OFF and ON. Note that there should be no flow with the ignition on

  1. Check that the valve is closed when the ignition switch is in the ON position.
  2.  
  3. Reconnect the hoses to the proper locations. If the valve doesn't operate correctly, double check the fuse and wiring before replacing the valve.
  4.  

Thermo Switch
NOVA WITH VIN CODE 4 ENGINE

See Figure 4

  1. Drain the coolant from the radiator into a clean container.
  2.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Remove the thermoswitch from the intake manifold. The switch is located behind the TVSV (Thermo Vacuum Switching Valve).
  2.  
  3. 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.
  4.  



Click image to see an enlarged view

Fig. Fig. 4: The thermoswitch should have continuity if cooled below 109 degrees F

  1. 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.
  2.  
  3. Apply sealer to the threads of the switch and reinstall it in the manifold.
  4.  
  5. Refill the radiator with coolant.
  6.  

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.

 
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