GM Corsica/Beretta 1988-1996 Repair Guide

Evaporative Emission Controls

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OPERATION



See Figures 1, 2, 3 and 4

The Evaporative Emission Control System (EECS) 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 inside the left rear wheel well. 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. Three different methods are used to control the purge cycle of the charcoal canister.

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. 1: Evaporative emission control system schematic - 1988-91 vehicles

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.
 



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Fig. Fig. 2: Cross-sectional view of the vapor canister - 1988 2.0L engines

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.



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Fig. Fig. 3: Some vehicles utilize a fuel vapor pressure control valve



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Fig. Fig. 4: The EVAP canister is usually mounted in the front corner engine compartment or behind the rear wheel liner

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



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 (51kPa) 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



Evaporative Canister

See Figures 5 and 6

  1. Disconnect the negative battery cable.
  2.  
  3. If necessary for access, remove the coolant recovery reservoir.
  4.  
  5. For 1996 3.1L engines, perform the following:
    1. Raise and safely support the vehicle, then remove the left (driver) side rear wheel and tire assembly.
    2.  
    3. Remove the left (driver) side rear wheel well liner.
    4.  

  6.  
  7. Tag and disconnect the vacuum hoses from the canister.
  8.  
  9. Unfasten the retainers, then remove the canister from the vehicle.
  10.  



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Fig. Fig. 5: On 1996 3.1L engines, you must remove the rear driver's side wheel well liner to access the canister



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Fig. Fig. 6: The evaporative canister is usually secured with 1 or 2 retaining bolts

To install:
  1. Position the canister into the vehicle, then secure with the retainers.
  2.  
  3. Attach the vacuum lines as tagged during removal.
  4.  
  5. For 1996 3.1L engines, perform the following:
    1. Fasten the left (driver) side rear wheel well liner.
    2.  
    3. Install the left rear wheel and tire assembly, then carefully lower the vehicle.
    4.  

  6.  
  7. If removed, install the coolant recovery reservoir.
  8.  
  9. Connect the negative battery cable.
  10.  

Tank Pressure Control Valve

See Figure 7

  1. Disconnect the negative battery cable.
  2.  
  3. Tag and disconnect the hoses from the control valve.
  4.  
  5. Remove the mounting hardware.
  6.  
  7. Remove the control valve from the vehicle.
  8.  



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Fig. Fig. 7: Cross-sectional view of the fuel tank pressure control valve

To install:
  1. Position the control valve in the vehicle, then secure using the mounting hardware.
  2.  
  3. Attach the hoses to the control valve, as tagged during removal.
  4.  
  5. Connect the negative battery cable.
  6.  

Canister Purge Solenoid Valve

See Figures 8

  1. Disconnect the negative battery cable.
  2.  
  3. If necessary for access, raise and safely support the vehicle.
  4.  



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Fig. Fig. 8: Exploded view of the evaporative canister purge solenoid valve mounting - 1996 2.2L engine shown

  1. Tag and detach the electrical connector and vacuum hose(s) from the valve.
  2.  
  3. Either unfasten the mounting nut or release the locktab on the solenoid bracket.
  4.  
  5. Remove the canister purge solenoid valve from the vehicle.
  6.  

To install:
  1. Position the purge solenoid valve to the bracket. Either snap the valve over the locktabs of the bracket or fasten the retaining nut to secure the valve.
  2.  
  3. Attach the vacuum hose(s) and electrical connector to the valve, as tagged during removal.
  4.  
  5. If necessary, carefully lower the vehicle.
  6.  
  7. Connect the negative battery cable.
  8.  

EVAP Vacuum Switch

See Figure 9

  1. Disconnect the negative battery cable.
  2.  
  3. Label and detach the switch electrical connector(s) and vacuum hoses.
  4.  
  5. Bend the retaining tab on the bracket to remove the switch, then remove the switch from the vehicle.
  6.  



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Fig. Fig. 9: Evaporative emission canister vacuum switch from a 2.2L engine shown

To install:
  1. Connect the vacuum hoses to the switch.
  2.  
  3. Position the vacuum switch on the bracket.
  4.  
  5. Bend the retaining tab to secure the switch to the bracket.
  6.  
  7. Attach the switch electrical connector.
  8.  
  9. Connect the negative battery cable.
  10.  

 
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