Honda Accord/Prelude 1984-1995 Repair Guide

Evaporative Emission Control System

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OPERATION



This type of system prevents gasoline vapors from escaping into the atmosphere from the fuel tank and carburetor (if equipped). The actual systems used on these cars will vary between vehicle families, based on year and equipment. Purge control (the admission of fresh air into the canister), may be accomplished either by mechanical means (temperature and vacuum operated controls) or electronically by the Engine Control Unit (ECU). Note that the use of computers is not limited solely to fuel injected vehicles.

The system works by storing fuel vapor in the fuel tank expansion chamber and in the vapor line. When system pressure becomes higher than the set value of the one-way valve, the valve opens and allows vapor into the charcoal canister. While the engine is stopped or idling, the idle cut-off valve or purge control system is closed and the vapor is absorbed by the charcoal.

At partially opened throttle, the idle cut-off valve is opened by manifold vacuum. The vapor that was stored in the charcoal canister and in the vapor line is purged into the intake manifold. Any excessive pressure or vacuum which might build up in the fuel tank is relieved by the two-way valve in the filler cap or fuel line.

TESTING



Carbureted 1984-85 Accord and Prelude
THERMOVALVE

See Figure 1

  1. Disconnect the upper hose (purge control diaphragm valve) from the evaporative canister. Connect the hose to a hand vacuum pump or vacuum gauge.
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Purge control diaphragm valve port

Coolant temperature must be below the thermovalve set temperature. To check this, use an ohmmeter to test for continuity across the thermovalve terminals. If it is below the set temperature, continuity should be present. If continuity is not present, the thermovalve may either be defective or the engine has not cooled sufficiently.

  1. Start the engine and allow it to idle; there should be no vacuum indicated on the gauge.
  2.  
  3. If there is vacuum, replace thermovalve A on Accords, or thermovalve B on Preludes, then retest. To locate the thermovalve, trace the vacuum line leading from the charcoal canister to the air cleaner.
  4.  

The engine must at normal operating temperature for the following steps. The cooling fan(s) must cycle on and off at least once.

  1. Allow the engine to idle; there should be vacuum indicated on the gauge.
  2.  
  3. If there is no vacuum, inspect thermovalve A on Accords or B on Preludes by pinching its hose leading to the air cleaner. If vacuum is now available to the canister, replace the thermovalve. If vacuum is still not available, inspect hose No. 19 to the intake manifold for leaks or blockage (refer to the vacuum diagrams in this section.)
  4.  

CHARCOAL CANISTER

See Figures 2, 3 and 4

The engine must at normal operating temperature for the following steps. The cooling fan(s) must cycle on and off at least once.

  1. Connect a tachometer to the engine.
  2.  
  3. Remove the fuel filler cap.
  4.  
  5. Disconnect the purge air hose from under the canister and connect a vacuum gauge to it.
  6.  



Click image to see an enlarged view

Fig. Fig. 2: Connect a vacuum gauge to the purge air hose on the bottom of the canister

  1. Start the engine.
  2.  
  3. Raise the engine speed to 3500 rpm; vacuum should appear on the gauge in 1 minute. If vacuum appears, proceed to the last test step.
  4.  
  5. If no vacuum appears:
    1. Stop the engine.
    2.  
    3. Disconnect the vacuum gauge and reinstall the fuel filler cap.
    4.  
    5. Remove the charcoal canister and check for signs of damage or defects.
    6.  
    7. If necessary, replace the canister.
    8.  

  6.  
  7. Disconnect the PCV hose from the charcoal canister.
  8.  
  9. Connect a vacuum pump to the PURGE fitting on the canister and apply vacuum. If the vacuum remains steady, proceed with the next step; if the vacuum drops, replace the canister and retest.
  10.  



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Fig. Fig. 3: Connect a vacuum pump to the PURGE fitting of the canister

  1. Reconnect the PCV hose and start the engine. The purge vacuum should drop to zero. If the vacuum does not drop to zero, replace the canister and retest.
  2.  
  3. If purge vacuum does drop to zero, connect the vacuum pump to the canister PCV fitting and draw vacuum; it should remain steady. If vacuum remains steady, disconnect the pump and recheck the operation of the thermovalve. If vacuum drops, replace the canister and retest.
  4.  
  5. Connect a hand vacuum pump to the TANK fitting on the canister and draw vacuum. No vacuum should be held. If there is no vacuum present, reinstall the fuel filler cap; the test is complete. If vacuum is held, replace the canister and retest.
  6.  



Click image to see an enlarged view

Fig. Fig. 4: Connect a vacuum pump to the TANK fitting on the charcoal canister

Fuel Injected 1985 Accord
CHARCOAL CANISTER

See Figures 2, 5, 6 and 7

  1. Connect a tachometer to the engine.
  2.  
  3. Remove the fuel filler cap.
  4.  
  5. Disconnect the purge air hose from under the canister and connect a vacuum gauge to it.
  6.  
  7. Start the engine and allow it to reach normal operating temperature; the cooling fan should cycle on and off at least once.
  8.  
  9. Raise the engine speed to 3500 rpm; vacuum should appear on the gauge in 1 minute. If vacuum appears, proceed to the last test step.
  10.  
  11. If no vacuum appears:
    1. Disconnect the vacuum gauge and reinstall the fuel filler cap.
    2.  
    3. Remove the charcoal canister and check for signs of damage or defects.
    4.  
    5. If necessary, replace the canister.
    6.  

  12.  
  13. Connect the vacuum pump to the canister PURGE fitting and apply vacuum; which should then remain steady once applied. If the vacuum drops, replace the canister and retest.
  14.  



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Fig. Fig. 5: Connect a vacuum pump to the PURGE port fitting on the charcoal canister

  1. Using a 2nd vacuum pump, draw a vacuum from the canister PCV fitting. Purge side vacuum should drop to zero. If vacuum does not drop, replace the canister and retest.
  2.  



Click image to see an enlarged view

Fig. Fig. 6: Use a 2nd vacuum pump to draw a vacuum from the canister PCV fitting

  1. Disconnect the vacuum pump from the PURGE port. Draw vacuum at the PCV port; steady vacuum should be held. If vacuum drops, replace the canister.
  2.  
  3. Connect the vacuum pump to the TANK port and draw vacuum. If no vacuum is present, reinstall the filler cap; the test is complete. If vacuum is present, replace the canister and retest.
  4.  



Click image to see an enlarged view

Fig. Fig. 7: No vacuum should be present at the TANK port

TWO-WAY VALVE

See Figure 8

  1. Remove the fuel filler cap.
  2.  
  3. Elevate and safely support the vehicle.
  4.  
  5. Remove the vapor line from the liquid/vapor separator pipe at the side of the fuel tank. Connect a T-fitting for both a vacuum gauge and vacuum pump to the hose running into the tank.
  6.  
  7. Slowly draw a vacuum while watching the gauge. Vacuum should stabilize as the two-way valve opens between 0.2-0.6 in. Hg (5-15 mm Hg). If the valve opens too early or too late, replace the two-way valve and retest.
  8.  



Click image to see an enlarged view

Fig. Fig. 8: To test the two-way valve, it is necessary to use a pump capable of supplying both vacuum and pressure

  1. Move the vacuum pump hose from the vacuum to the pressure fitting on the pump.
  2.  
  3. Slowly pressurize the vapor line while watching the gauge. Pressure should stabilize between 1.0-2.2 in Hg. (25-55 mm Hg). If this is true, the valve is OK. If the pressure stabilizes too early or too late, replace the valve.
  4.  

Carbureted 1986-89 Accords and 1986-90 Preludes
THERMOVALVE

See Figures 9, 10 and 11

The engine coolant temperature must be below 131°F (55°C) on Accords or 104°F (40°C) on Preludes.

  1. Disconnect the vacuum hose at the purge control diaphragm valve on the top of the canister. Connect a vacuum pump to the hose.
  2.  



Click image to see an enlarged view

Fig. Fig. 9: Purge control diaphragm valve vacuum hose fitting location

  1. Start the engine, allowing it to idle. Vacuum should not be available. If vacuum is present, replace the thermovalve and retest.
  2.  
  3. Allow the engine to reach normal operating temperature. Vacuum should be present. If no vacuum is present, disconnect the vacuum hose at thermovalve and check for vacuum. If vacuum is now present, replace the thermovalve.
  4.  
  5. Shut the engine OFF .
  6.  
  7. Disconnect the vacuum pump, then reconnect the hose.
  8.  
  9. Remove the fuel filler cap.
  10.  
  11. Remove the purge air hose from under the canister, then connect a vacuum gauge to it.
  12.  



Click image to see an enlarged view

Fig. Fig. 10: Connect a vacuum gauge to the canister purge air hose from under the canister or on the frame (as applicable)

  1. Start the engine, then raise the speed to 3500 rpm. Allow the engine to run for 1 minute. Vacuum should appear on the gauge. If no vacuum is present, remove the charcoal canister and inspect for physical damage. Replace it if necessary. If vacuum is present, proceed to the next step.
  2.  
  3. Disconnect the hose from the canister PCV fitting. Connect a vacuum pump to the PURGE fitting and apply vacuum, which should remain steady. If vacuum drops, replace the canister.
  4.  



Click image to see an enlarged view

Fig. Fig. 11: PCV and PURGE fitting locations on the canister

  1. Connect the PCV hose to the fitting, then restart the engine. Vacuum at the purge port should drop to 0. If not, replace the canister. If it does, continue with the next step.
  2.  
  3. Connect a vacuum pump to the canister PCV fitting and apply vacuum, which should hold steady. If the vacuum does not hold, replace the canister and retest.
  4.  
  5. Connect the vacuum pump to the TANK fitting on the canister and apply vacuum, which should NOT be held.
  6.  

If vacuum is present, replace the canister.

CARBURETOR THERMOVALVE

See Figure 12

This test procedure applies to the Accord only.

  1. Begin the test on a cold engine; the coolant temperature must be below 86°F (30°C).
  2.  
  3. Disconnect the hose at the carburetor thermovalve, then connect a hand vacuum pump to the valve. Draw a vacuum of 8 in. Hg (200 mm Hg) to the valve; the vacuum should remain steady. If the vacuum cannot be held, replace the thermovalve.
  4.  



Click image to see an enlarged view

Fig. Fig. 12: A hand vacuum pump is used to test the carburetor thermovalve

  1. Reconnect the vacuum hose to the carburetor thermovalve. Start the engine, allowing it to warm up to normal operating temperature. Coolant temperature must be above 104°F (40°C).
  2.  
  3. Repeat the test. No vacuum should be held. If vacuum is held, replace the carburetor thermovalve.
  4.  

Fuel Injected 1986-89 Accords and 1986-91 Preludes
SYSTEM TEST

See Figure 10

Begin the test on a cold engine. Coolant temperature must be below 131°F (55°C).

  1. Inspect the vacuum lines for cracks, blockages and proper connections.
  2.  
  3. Disconnect the No. 3 vacuum hose (refer to the vacuum diagrams later in this section) from the purge control diaphragm valve and connect a vacuum gauge to the hose.
  4.  
  5. Start the engine and allow it to idle. While the engine is cold, vacuum should not be available. If vacuum is present, replace the thermovalve and retest.
  6.  
  7. Allow the engine to warm up to normal operating temperature. Vacuum should become available as the engine coolant passes 131°F (55°C). If vacuum is not present, replace the thermovalve.
  8.  
  9. Turn the engine OFF .
  10.  
  11. Disconnect the vacuum gauge and reconnect the hose.
  12.  
  13. Remove the fuel filler cap.
  14.  
  15. Remove the canister purge air hose from the frame and connect the hose to a vacuum gauge.
  16.  
  17. Start the engine, then raise the speed to 3500 rpm. Allow the engine to run for 1 minute. Vacuum should appear on the gauge. If no vacuum is present, remove the charcoal canister and inspect for physical damage. Replace it if necessary. If vacuum is present, proceed to the next step.
  18.  
  19. Disconnect the hose from the canister PCV fitting. Connect a vacuum pump to the PURGE fitting and apply vacuum, which should remain steady. If vacuum drops, replace the canister.
  20.  
  21. Connect the PCV hose to the fitting, then restart the engine. Vacuum at the purge port should drop to 0. If not, replace the canister. If it does, continue with the next step.
  22.  
  23. Connect a vacuum pump to the canister PCV fitting and apply vacuum. Steady vacuum should be held. If not, replace the canister and retest.
  24.  
  25. Connect the vacuum pump to the TANK fitting on the canister and draw vacuum. No vacuum should be held; if vacuum is present, replace the canister.
  26.  

TWO-WAY VALVE

See Figure 13

  1. Remove the fuel filler cap.
  2.  
  3. Elevate and safely support the vehicle.
  4.  
  5. Remove the vapor line from the liquid/vapor separator pipe at the side of the fuel tank. Connect a T-fitting for both a vacuum gauge and vacuum pump to the hose running into the tank.
  6.  
  7. Slowly draw a vacuum while watching the gauge. The vacuum should stabilize as the two-way valve opens at between 0.2-0.6 in. Hg (5-15 mm Hg). If the valve opens too early or too late, replace the two-way valve and retest.
  8.  



Click image to see an enlarged view

Fig. Fig. 13: To test the two-way valve, it is necessary to use a pump capable of supplying both vacuum and pressure

  1. Move the vacuum pump hose from the vacuum to the pressure fitting on the pump. If necessary, move the gauge hose to the pressure side.
  2.  
  3. Slowly pressurize the vapor line while watching the gauge. Pressure should stabilize between 1.0-2.2 in Hg. (25-55 mm Hg). If this is true, the valve is OK. If the pressure stabilizes too early or too late, replace the valve.
  4.  

1990-95 Accord and 1992-95 Prelude
TWO-WAY VALVE

See Figure 14

  1. Remove the fuel filler cap.
  2.  
  3. Elevate and safely support the vehicle.
  4.  
  5. Remove the vapor line from the liquid/vapor separator pipe at the side of the fuel tank. Connect a T-fitting for both a vacuum gauge and vacuum pump to the hose running into the tank.
  6.  
  7. Slowly draw a vacuum while watching the gauge. Vacuum should stabilize as the two-way valve opens between 0.2-0.6 in. Hg (5-15 mm Hg). If the valve opens too early or too late, replace the two-way valve and retest.
  8.  
  9. Move the vacuum pump hose from the vacuum to the pressure fitting on the pump. If necessary, move the gauge hose to the pressure side.
  10.  



Click image to see an enlarged view

Fig. Fig. 14: Two-way valve testing

  1. Slowly pressurize the vapor line while watching the gauge. Pressure should stabilize between 1.0-2.2 in Hg. (25-55 mm Hg). If this is true, the valve is OK. If the pressure stabilizes too early or too late, replace the valve.
  2.  

REMOVAL & INSTALLATION



See Figure 15

Removal and installation of the various evaporative emission control system components consists of labeling or marking and unfastening hoses, loosening retaining screws, and removing the part which is to be replaced from its mounting point.



Click image to see an enlarged view

Fig. Fig. 15: The charcoal canister is commonly "hidden" near the firewall in the engine compartment

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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