Chevrolet Full Size Cars 1979-1989

Evaporative Emission Control

Print

OPERATION





Click image to see an enlarged view

Fig. Fig. 1 The evaporative emission canister is usually mounted in the side of the engine compartment



Click image to see an enlarged view

Fig. Fig. 2 Evaporative emission control system schematic for early model "open" design canisters



Click image to see an enlarged view

Fig. Fig. 3 Common evaporative emission control schematic for Chevrolet 5.0L engines equipped with thermal vacuum control



Click image to see an enlarged view

Fig. Fig. 4 Common closed canister evaporative control schematic for carbureted vehicles equipped with a vacuum solenoid



Click image to see an enlarged view

Fig. Fig. 5 Evaporative emission control schematic fuel injected vehicles

All gasoline vehicles covered by this guide are equipped with an evaporative emission control system which is designed to reduce the amount gasoline vapors which escape into the atmosphere. Float bowl emissions are controlled by internal carburetor modifications and, on later model vehicles, by a vapor line to the canister. Redesigned bowl vents, reduced bowl capacity, heat shields and improved intake manifold-to-carburetor (or throttle body on fuel injected vehicles) insulation reduce vapor loss. The venting of fuel tank vapors into the air has been stopped by means of the carbon canister storage method. This method transfers fuel vapors to an activated carbon storage device which absorbs and stores the vapor that is emitted from the engine's induction system while the engine is not running. When the engine is running, the stored vapor is purged from the carbon storage device by the intake air flow and then consumed in the normal combustion process. The system, in its simplest form, works when manifold vacuum reaches a certain point and opens a purge control valve mounted atop or near the charcoal storage canister. This allows air to be drawn into the canister, thus forcing the existing fuel vapors back into the engine to be burned normally.

The purge function on most earlier model vehicles was controlled by a Thermal Vacuum Switch (TVS) located inline between the canister and the carburetor/intake manifold. The thermal vacuum switch was threaded into a coolant passage such as the thermostat housing and would be activated by engine coolant temperature. As the engine warmed, the switch would open to allow vacuum to the canister or canister control valve.

Later vehicles switched from thermal to electronic control. The purge control on the 231 and on all fuel injected engines is electronically controlled by a normally opened inline purge solenoid which is itself activated by the Electronic Control Module (ECM). On the 231 and most fuel injection systems through 1987, when the system is in the Open Loop mode, the solenoid valve is energized, blocking all vacuum to the purge valve. When the system is in the Closed Loop mode, the solenoid is de-energized, thus allowing existing vacuum to operate the purge valve. This releases the trapped fuel vapor and it is forced into the induction system.

For almost all 1988-89 fuel injected vehicles, a new purge control solenoid was used. This solenoid was a Normally Closed (N/C) component which worked on opposite signals to its predecessor. On these vehicles the ECM would de-energize the solenoid during cold engine operation or idle conditions. When de-energized the solenoid would block all vacuum preventing canister purge. Once the engine was fully warmed and operated above idle, the ECM would energize the solenoid, allowing vacuum to purge the canister of stored fuel vapors.

Many of the carbureted vehicles covered by this guide are equipped with a float bowl vent to the canister. On these vehicles a vacuum valve is used to prevent vapor purge from the float bowl when the engine is running. Whenever the engine is off, the valve allows vapors to travel from the float bowl to the canister.

Though a few of the earlier vehicles covered here were equipped with carbon canisters of the "Open" design, meaning that air is drawn in through the bottom (filter) of the canister, most are equipped with a "Closed" design canister which uses a sealed bottom. On later model vehicles equipped with "Closed" design canisters, incoming air which is drawn directly from the air cleaner.

SERVICE



Besides a periodic visual inspection of the system's components, the only periodic service necessary (on early model vehicles so equipped) is canister filter replacement. Later vehicles are equipped with a sealed canister that is not equipped with a replaceable cartridge. On these vehicles, the entire canister assembly must be replaced if any damage occurs or any problems are found with the canister itself.

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

Periodically check for cracks or leaks in the vacuum lines or in the canister itself. The lines and fittings can usually be reached without removing the canister. Cracks or leaks in the system may cause poor idle, stalling, poor driveability, fuel loss or a fuel vapor odor.

Vapor odor and fuel loss may also be caused by; fuel leaking from the lines, tank or injectors, loose, disconnected or kinked lines or an improperly seated air cleaner and gasket.

If the system passes the visual inspection and a problem is still suspected, check the basic operation of the components:

  1. The line from the fuel tank to the canister must be clear and unobstructed. When the engine is OFF , air should pass from the fuel tank towards the canister freely in order to allow vapors to collect in the canister. Make sure the line is free of kinks or obstructions. While the engine is not running, air should NOT be allowed out of the canister.
  2.  
  3. If equipped with a float bowl vent, the vacuum valve should only allow air to be blown from the carburetor float bowl towards the canister when no vacuum is applied (engine is not running). To test this valve, attempt to blow air through the valve towards the canister with the engine OFF , there should be little or no restriction. Use a hand vacuum pump to apply approximately 15 in. Hg (51 kPa) to the valve, now air should no longer flow towards the canister.
  4.  
  5. Thermal valves are usually designed to open, allowing vacuum or air pressure towards the canister or control valve only when the engine is warm. Attach a length of hose to the engine side fitting and try blowing towards the canister. Air should be felt at the canister side fitting, only when the engine is at normal operating temperature.
  6.  

When testing valves by blowing air through them, be careful that you are blowing in the proper direction of flow. Many valves are designed to only allow air to flow in one direction and a proper working valve may seem defective if it is tested with air flow only in the wrong direction.

  1. Normally open solenoid valves, which are used on some engines before 1988, should close when energized and open when de-energized. Try blowing air through the valve fittings when the engine is OFF , it should flow with little or no resistance. When the engine is running the solenoid should energize during engine warm-up and de-energize once it has reached normal operating temperature.
  2.  
  3. Normally closed solenoid valves, used on most 1988-89 engines, should open when energized and close when deenergized. Try blowing air through the valve fittings when the engine is OFF , air should not flow. When the engine is running the solenoid should de-energize during engine warm-up and energize once it has reached normal operating temperature.
  4.  
  5. Most vacuum and control valves, with the exception of the float bowl valve, are designed to open when vacuum is applied. In either case, all are designed to allow air to pass through only during one condition (vacuum on or off depending on design). To test vacuum valves, try to blow air through the valve with and without vacuum applied. If air can pass through during only 1 of these conditions the valve is likely operating properly. On the other hand if air can always or never flow, the valve is defective. Use a hand vacuum pump to apply approximately 15 in. Hg (51 kPa) to the valve. The valve should open or close (as applicable) and hold the vacuum for at least 20 seconds, or the diaphragm is leaking and the valve must be replaced.
  6.  

REMOVAL & INSTALLATION



Canister
  1. If the hoses can be reached at this time, tag and disconnect them from the canister assembly.
  2.  
  3. Loosen the screw(s) fastening the canister retaining bracket to the vehicle.
  4.  
  5. Rotate the bracket and carefully remove the canister assembly.
  6.  
  7. If not done already, reposition the canister for access, then tag and disconnect the hoses.
  8.  
  9. Remove the canister assembly from the vehicle.
  10.  

To install:

  1. Position the canister in the vehicle.
  2.  
  3. If the hoses cannot be accessed once the canister is secured, connect them now as noted during removal.
  4.  
  5. Install the canister to the retaining bracket and secure using the retaining screw(s).
  6.  
  7. If not done earlier, connect the lines to the canister assembly as noted during removal.
  8.  

Filter

Some early model vehicles are equipped with an "Open" canister which uses a replaceable filter.

  1. Remove the vapor canister from the vehicle.
  2.  
  3. Grasp the filter in the bottom of the canister with your fingers, then squeeze and pull it out from under the lip surface or from under the retainer bar.
  4.  

To install:

  1. Install a new filter to the bottom of the canister by squeezing and carefully inserting the filter into the canister. Make sure the edges are tucked under the canister lip.
  2.  
  3. Install the canister assembly to the vehicle.
  4.  

 
label.common.footer.alt.autozoneLogo