GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

Evaporative Emission Control System

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OPERATION



See Figures 1 and 2

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 in a charcoal canister, mounted under the hood. 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. A few different methods (depending upon application) are used to control the purge cycle of the charcoal canister.



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Fig. Fig. 1: Cross-sectional view of a typical EVAP canister

In the first method, 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, a vacuum is created in the throttle body venturi. That vacuum acts on the canister causing fresh air to be drawn into the bottom of the canister and the fuel vapors to be carried into the throttle body at that vacuum port. The air/vapor flow volume is only what can be drawn through the vacuum port and is fairly constant.



Click image to see an enlarged view

Fig. Fig. 2: Schematic of the evaporative emissions control system

For the second method, 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 computer control module through a solenoid valve mounted on or remotely from 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 for a specified period of time.
 
Vehicle speed is above a predetermined speed.
 
Throttle opening is above a predetermined value.
 

Remember that the fuel tank filler cap is an integral part of the system in that is was designed to seal in fuel vapors. If it is lost or damaged, make sure the replacement is or the correct size and fit so a proper seal can be obtained.

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 0.73 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 not liquid fuel leak, check for a cracked or damaged vapor canister, inoperative or always open canister control valve, disconnected, mis-routed, 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 the 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 purge control 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



Charcoal Canister

On some models it may be necessary to remove the right side fender fascia and filler panels to gain access to the canister.

  1. Disconnect the negative battery cable.
  2.  
  3. Tag and disconnect the hoses from the canister.
  4.  
  5. Unfasten the charcoal canister retaining nuts/bolts and remove any retaining straps that may be securing the canister.
  6.  
  7. If necessary for access, and equipped with air conditioning, loosen the attachments holding the accumulator and pipe assembly.
  8.  
  9. Remove the canister from the vehicle.
  10.  

Always replace any vapor hose(s) that may be showing signs of wear.

  1. Installation is the reverse of the removal procedure. Refer to the Vehicle Emission Control Information label, located in the engine compartment, for proper routing of the vacuum hoses.
  2.  

Tank Pressure Control Valve
  1. Tag and disconnect the hoses from the control valve.
  2.  
  3. Unfasten the mounting hardware.
  4.  
  5. Remove the control valve from the vehicle.
  6.  
  7. Installation is the reverse of the removal procedure. Refer to the Vehicle Emission Control Information label, located in the engine compartment, for proper routing of the vacuum hoses.
  8.  

Canister Purge Control Solenoid Valve
  1. Disconnect the negative battery cable.
  2.  
  3. Tag and detach the electrical connector(s), hose and line from the solenoid valve.
  4.  
  5. Unfasten the retainer (screw, bolts or locking tab), then remove the valve from the vehicle.
  6.  
  7. Installation is the reverse of the removal procedure.
  8.  

Filter Replacement
  1. Remove the vapor canister, as outlined earlier in this section.
  2.  
  3. Pull the filter out from the bottom of the canister.
  4.  
  5. Install a new filter and then replace the canister.
  6.  

 
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