Toyota Previa 1991-1997 Repair Information

Evaporative Emission Controls

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OPERATION



The Evaporative Emission Control (EVAP) system is designed to prevent fuel tank vapors from being emitted into the atmosphere. When the engine is not running, gasoline vapors from the tank are stored in a charcoal canister. The charcoal canister absorbs the gasoline vapors and stores them until certain engine conditions are met and the vapors can be purged and burned by the engine. In some vehicles, any liquid fuel entering the canister goes into a reservoir in the bottom of the canister to protect the integrity of the carbon element in the canister above. These systems employ the following components:



Fuel tank cap
 
Charcoal canister
 
Check valve
 
Vacuum Switching Valve (VSV)-2TZ-FZE only
 

COMPONENT TESTING



See Figure 1

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.



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Fig. Fig. 1: Always inspect the lines for kinks, cracks and loose connections

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

See Figure 2

Check that the filler cap seals effectively. Replace the filler cap if the seal is defective.



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Fig. Fig. 2: Inspect the gasket on the fuel cap, if deteriorated, replace the gasket or cap as necessary

Charcoal Canister

See Figures 3 and 4

  1. Remove the charcoal canister from the vehicle.
  2.  
  3. Remove the cap from the canister.
  4.  
  5. Visually check the charcoal canister for cracks or damage.
  6.  
  7. Check for a clogged filter and stuck check valve. Using low pressure compressed air (0.68 psi. or 4 kPa), 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.
  8.  



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Fig. Fig. 3: To check for a clogged filter or check valve, blow compressed air into the pipes as shown

  1. Next blow air into the purge pipe and check that air does not flow from the other pipes.
  2.  
  3. Clean the filter in the canister by blowing no more than 43 psi (294 kPa) of compressed air into the purge pipe to the outer vent control valve while holding the purge pipe closed.
  4.  



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Fig. Fig. 4: To clean the filter blow compressed air into the tank pipe while holding the purge pipe 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.

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

Check Valve

See Figures 5 and 6

  1. Remove the check valve from the engine.
  2.  
  3. Blow air into the orange side of the pipe and check for air flow from the orange side.
  4.  



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Fig. Fig. 5: Testing the check valve for air flow on each side

  1. Blow air into the orange side of the pipe and check that no air flows from the black side.
  2.  
  3. If the testing is not as specified, replace the check valve.
  4.  
  5. Reinstall the check valve with the orange side facing the No. 2 air inlet duct side.
  6.  



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Fig. Fig. 6: Only install the check valve with the orange side facing the No. 2 air inlet duct

Vacuum Switching Valve (VSV)
filterED type

See Figures 7, 8, 9 and 10

The VSV is attached to the No. 2 air inlet duct. Some VSV models are equipped with an air filter at the end.

  1. Check that air flows from port E to the air filter.
  2.  



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Fig. Fig. 7: Check that air flows from port E to the air filter of the VSV

  1. Connect the vacuum switching valve terminals to the battery.
  2.  



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Fig. Fig. 8: Apply battery voltage across the terminals of the VSV

  1. Check that air flows from port E to port F .
  2.  

  1. If the VSV fails this test replace it. Any doubts perform the following test.
  2.  
  3. Remove the VSV.
  4.  
  5. Check for an open circuit. Using an ohmmeter, measure the resistance (ohms) between the two terminals of the valve. The resistance (cold) should be 30-34 ohms. If the resistance is not within specifications, replace the VSV.
  6.  



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Fig. Fig. 9: Check for open ground on the VSV using an ohmmeter

  1. Check for a short circuit within the valve. Using an ohmmeter, check that there is no continuity between the terminals and the VSV body. If there is continuity, replace the VSV.
  2.  



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Fig. Fig. 10: Check for ground on the VSV using an ohmmeter

Non-FilterED Type
  1. Check that air does not flow from port E to port F .
  2.  
  3. Connect the vacuum switching valve terminals to the battery.
  4.  
  5. Check that air flows from port E to port F .
  6.  
  7. If the VSV fails this test replace it. Any doubts perform the following test.
  8.  
  9. Remove the VSV.
  10.  
  11. Check for an open circuit. Using an ohmmeter, measure the resistance (ohms) between the two terminals of the valve. The resistance (cold) should be 30-34 ohms. If the resistance is not within specifications, replace the VSV.
  12.  
  13. Check for a short circuit within the valve. Using an ohmmeter, check that there is no continuity between the terminals and the VSV body. If there is continuity, replace the VSV.
  14.  

REMOVAL & INSTALLATION



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.

Charcoal Canister

See Figures 11, 12, 13, 14 and 15

Label and disconnect the lines running to the canister. Make sure to plug the lines with a bolt or like. Disconnect the wiring harness. Unbolt and remove the charcoal canister from the vehicle. Do not attempt to wash the charcoal canister. Also be sure that no activated carbon comes out of the canister during the cleaning process. Attach the charcoal canister to its mounting bracket and secure. Connect the vacuum hoses in their proper locations.



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Fig. Fig. 11: Remove the hose clamp for the lower tube on the charcoal canister



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Fig. Fig. 12: Disconnect the wiring harness leading to the canister



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Fig. Fig. 13: Remove the two canister mounting bolts



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Fig. Fig. 14: Always plug the hoses to prevent gas vapors from escaping



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Fig. Fig. 15: View of the common charcoal canister

Vacuum Switching Valve (VSV)

See Figure 16

Disconnect the wiring from the VSV. Label and removed the vacuum hoses from the valve. Loosen the nut and extract the VSV from the No. 2 air inlet duct.



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Fig. Fig. 16: The Vacuum Switching Valve is attached to the No. 2 air inlet duct

 
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