GM Corvette 1984-1996 Repair Guide

Evaporative Emission Controls

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OPERATION



See Figures 1, 2 and 3

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, mounted either in the left front of the engine compartment on 1984-90 vehicles, or mounted inside the right rear wheel well on 1991-96 vehicles. 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 with fuel injection, 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. Three different methods are used to control the purge cycle of the charcoal canister.



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Fig. Fig. 1: On 1984-90 vehicles, the canister is mounted in the front drivers side corner of the engine compartment



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Fig. Fig. 2: The canister is located inside the right rear wheel well on 1991-96 vehicles

First, the charcoal canister purge cycle is controlled by throttle position without the use of a valve on the canister. A vacuum line connects the canister to a ported vacuum source on the throttle body. When the throttle is at any position above idle, fresh air is drawn into the bottom of the canister and the fuel vapors are carried into the throttle body at that port. The air/vapor flow volume is only what can be drawn through the vacuum port and is fairly constant.



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Fig. Fig. 3: Typical evaporative emission control system schematic

Second, the flow volume is modulated with throttle position through a vacuum valve. The ported vacuum from the throttle body is used to open a diaphragm valve on top of the canister. When the valve is open, air and vapors are drawn into the intake manifold, usually through the same manifold port as the PCV system. With this method, the purge valve cycle is slaved to the throttle opening; more throttle opening, more purge air flow.

And third, the charcoal canister purge valve cycle is controlled by the ECM through a solenoid valve on the canister. When the solenoid is activated, full manifold vacuum is applied to the top of the purge valve diaphragm to open the valve all the way. A high volume of fresh air is drawn into the canister and the gasoline vapors are purged quickly. The ECM activates the solenoid valve when the following conditions are met:



The engine is at normal operating temperature.
 
After the engine has been running a specified period of time.
 
Vehicle speed is above a predetermined speed.
 
Throttle opening is above a predetermined value.
 

A vent pipe allows fuel vapors to flow to the charcoal canister. On some vehicles, the tank is isolated from the charcoal canister by a tank pressure control valve, located either in the tank or in the vapor line near the canister. It is a combination roll-over, integral pressure and vacuum relief valve. When the vapor pressure in the tank exceeds 0.72 psi (5 kPa), the valve opens to allow vapors to vent to the canister. The valve also provides vacuum relief to protect against vacuum build-up in the fuel tank and roll-over spill protection.

Poor engine idle, stalling and poor driveability can be caused by an inoperative canister purge solenoid, a damaged canister or split, damaged or improperly connected hoses.

The most common symptom of problems in this system is fuel odors coming from under the hood. If there is no liquid fuel leak, check for a cracked or damaged vapor canister, inoperative or always open canister control valve, disconnected, misrouted, kinked or damaged vapor pipe or canister hoses; or a damaged air cleaner or improperly seated air cleaner gasket.

TESTING



Charcoal Canister
  1. Visually check the canister for cracks or damage.
  2.  
  3. If fuel is leaking from the bottom of the canister, replace canister and check for proper hose routing.
  4.  
  5. Check the filter at the bottom of the canister. If dirty, replace the filter.
  6.  

Tank Pressure Control Valve
  1. Using a hand-held vacuum pump, apply a vacuum of 15 in. Hg (51 kPa) through the control vacuum signal tube to the purge valve diaphragm. If the diaphragm does not hold vacuum for at least 20 seconds, the diaphragm is leaking. Replace the control valve.
  2.  
  3. With the vacuum still applied to the control vacuum tube, attach a short piece of hose to the valve's tank tube side and blow into the hose. Air should pass through the valve. If it does not, replace the control valve.
  4.  

Canister Purge Control Valve
  1. Connect a clean length of hose to the fuel tank vapor line connection on the canister and attempt to blow through the purge control valve. It should be difficult or impossible to blow through the valve. If air passes easily, the valve is stuck open and should be replaced.
  2.  
  3. Connect a hand-held vacuum pump to the top vacuum line fitting of the purge control valve. Apply a vacuum of 15 in. Hg (51 kPa) to the purge valve diaphragm. If the diaphragm does not hold vacuum for at least 20 seconds the diaphragm is leaking. Replace the control valve. If it is impossible to blow through the valve, it is stuck closed and must be replaced.
  4.  
  5. On vehicles with a solenoid activated purge control valve, unplug the connector and use jumper wires to supply 12 volts to the solenoid connections on the valve. With the vacuum still applied to the control vacuum tube, the purge control valve should open and it should be easy to blow through. If not, replace the valve.
  6.  

REMOVAL & INSTALLATION



EVAP Canister

See Figures 4, 5 and 6

  1. Disconnect the negative battery cable.
  2.  
  3. For 1991-96 vehicles, perform the following:
    1. Raise and safely support the vehicle.
    2.  
    3. Remove the right rear tire and wheel assembly.
    4.  
    5. Remove the canister splash shield or access panel.
    6.  

  4.  
  5. Tag and disconnect the vacuum hoses from the canister.
  6.  
  7. Loosen the screw or release the latch securing the canister retaining bracket.
  8.  
  9. Remove the canister from the vehicle.
  10.  



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Fig. Fig. 4: EVAP canister location and mounting-1990 vehicle shown



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Fig. Fig. 5: The EVAP canister and purge solenoid/valve are secured to a mounting bracket-1990 vehicle shown

To install:
  1. Install the canister into the retaining bracket.
  2.  
  3. Tighten the screw or fasten the latch securing the canister retaining bracket, as applicable.
  4.  
  5. Connect the hoses to the canister as tagged during removal.
  6.  
  7. For 1991-96 vehicles, perform the following:
    1. Install the access panel or canister splash shield.
    2.  
    3. Install the right rear tire and wheel assembly, then carefully lower the vehicle.
    4.  

  8.  
  9. Connect the negative battery cable.
  10.  

EVAP Canister Purge Solenoid/Valve
1984-90 VEHICLES

See Figure 5

  1. Disconnect the negative battery cable.
  2.  
  3. Unfasten the bolt, then remove the cover and solenoid.
  4.  
  5. Detach the solenoid electrical connector, then remove the solenoid from the vehicle.
  6.  

To install:
  1. Install the solenoid, cover and retaining bolt. Tighten the bolt to 51 inch lbs. (6 Nm).
  2.  
  3. Attach the hoses and electrical connector to the solenoid.
  4.  
  5. Connect the negative battery cable.
  6.  

1991-96 VEHICLES

See Figure 6

  1. Disconnect the negative battery cable.
  2.  
  3. If necessary, remove the fastener from the ignition test connector.
  4.  
  5. Detach the electrical connector and hoses from the solenoid valve.
  6.  
  7. Remove the bracket and solenoid from the intake manifold.
  8.  



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Fig. Fig. 6: Exploded view of the canister purge solenoid valve mounting-1991-96 vehicles

To install:
  1. Install the solenoid valve on the intake manifold. Install the retaining bolt to the manifold and tighten to 52 inch lbs. (5.8 Nm).
  2.  
  3. Attach the electrical connector and hoses to the solenoid valve.
  4.  
  5. If removed, install the ignition test connector to the bracket.
  6.  
  7. Connect the negative battery cable.
  8.  

EVAP Purge Vacuum Switch
1996 VEHICLES

See Figures 7 and 8

  1. Disconnect the negative battery cable.
  2.  
  3. Detach the switch electrical connector.
  4.  
  5. Disconnect the vacuum lines from the EVAP vacuum switch and the vacuum control solenoid.
  6.  



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Fig. Fig. 7: Disconnect the vacuum lines (4) from the vacuum switch (3) and the vacuum control solenoid (1)

  1. Remove the EVAP vacuum switch from the bracket.
  2.  



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Fig. Fig. 8: Remove the EVAP vacuum switch (2) from the bracket (4)

To install:
  1. Install the EVAP vacuum switch on the bracket.
  2.  
  3. Connect the vacuum lines to the vacuum switch and control solenoid.
  4.  
  5. Attach the electrical connector.
  6.  
  7. Connect the negative battery cable.
  8.  

 
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