GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Evaporative Emission Control



See Figures 1 and 2

This system which was introduced to California cars in 1970, and other cars in 1971, reduces the amount of escaping gasoline vapors. Float bowl emissions are controlled by internal carburetor modifications and, in some later vehicles, by a vapor line to the canister. Redesigned bowl vents, reduced bowl capacity, heat shields, and improved intake manifold-to-carburetor insulation reduce vapor loss into the atmosphere. 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. As the manifold vacuum reaches a certain point, it 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.

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

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Fig. Fig. 2: Certain engine adjustments, such a carburetor idle speed, often require disconnecting and plugging the canister purge line

The purge function on many 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 or the thermostat housing and would be activated by engine coolant temperature. Later vehicles switched from thermal to electronic control. The purge control on the 231 and 262 (1985-88) V6 engines is electronically controlled by an inline purge solenoid which is itself activated by the Electronic Control Module (ECM). 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.

On late model vehicles that are equipped with a float bowl vent to the canister, 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.

Most carbon canisters used are of the "Open'' design, meaning that air is drawn in through the bottom (filter) of the canister. Some 231 V6 canisters are of the "Closed'' design which means that the incoming air is drawn directly from the air cleaner.


See Figures 3, 4 and 5

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.

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Fig. Fig. 3: Early evaporative emission system schematic-open canister design

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.
  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.
  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.

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Fig. Fig. 4: Late model evaporative emission system schematic-closed canister design

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Fig. Fig. 5: Evaporative emission control system-5.0L engine

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. Solenoid valves, used on some late model engines, 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