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Diagnosing EVAP systems

Saturday, October 1, 2016 - 06:00
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Perhaps one of the toughest jobs in the automotive field is diagnosing small leaks, especially A/C and Fuel Vapor (EVAP) leaks. Finding these leaks can be time consuming and frustrating to say the least. Small EVAP system leaks may be the most difficult to locate because of several issues. First, the system test pressure is low at less than 1 PSI. Second, the leak sites that you must find are small at .005 to .030 of an inch. You may think you will not need to find these small .005 leaks, however you do. These small leaks can become large leaks with thermal expansion and contraction. The third issue is the EVAP system is located from one end of the vehicle to the other end of the vehicle. All of these are serious concerns and will require you to have a great understanding of EVAP systems and the test equipment used in order to find these elusive leak sites.  

Why do we have EVAP systems on vehicles? In southern California air quality became an issue in the mid 1960s. In order to control the air quality for human health, emission regulations were forced on the automotive industries. California established a new organization called the California Air Resource Board (CARB) in 1967. CARB is the “Clean Air Agency" of California. This organization became the first to impose requirements on the automotive industry and still leads the nation on regulating vehicle pollutants. After CARB was established, the Environmental Protection Agency (EPA) was established in 1970 to regulate pollutants in the United States that harm human health.

At first these regulations covered the internal combustion engine’s tailpipe and crankcase emissions and in 1970 moved to regulate the fuel handling and containment system (EVAP), which includes the engine’s tailpipe, crankcase emissions and fuel vapor produce smog — an acronym for smoke and fog. Photochemical smog is a type of air pollution caused by chemical reactions that occur between the sun’s ultraviolet light and pollutants such as hydrocarbons and oxides of nitrogen. Since these pollutants, referred to as smog, are created in part by hydrocarbons in the atmosphere reacting with sunlight, it has become necessary to prevent these hydrocarbons from entering the atmosphere.

Controlling fuel vapor

In order to accomplish reduced hydrocarbons, the fuel handling and containment system was required to trap the fuel vapor and then pull this trapped fuel vapor into the engine to be burned. To trap the fuel vapor (gasoline), activated carbon is used (Fig. 1). The carbon canister provides a bed of activated carbon that allows absorption and desorption of the many different species of hydrocarbons, which are contained in gasoline. These hydrocarbons are caught in the cracks in the activated carbon when the fuel handling system is venting hydrocarbons under atmospheric pressure. The activated carbon will adsorb these fuel vapors until it becomes saturated. In this saturated condition the activated carbon cannot take on any more hydrocarbons, thus the hydrocarbons go into the atmosphere. In order for this not to occur, the activated carbon must have a desorption cycle. This is accomplished by the purge cycle and is controlled by the purge valve. The hydrocarbons are then purged (pulled out) from the activated carbon using negative pressure applied by the running engine. An early EVAP systems (Fig. 2) were controlled by mechanical valving.  

Figure 1
Figure 2

This purge control valve is normally closed and is opened to allow atmospheric air to enter the running engine through the carbon canister vent. As the air moves through the vent and the carbon canister, it removes the hydrocarbons from the activated carbon. This air and hydrocarbon mixture is then moved into the running engine and burned. Since this mixture of air and hydrocarbons varies, it can affect the drivability of the engine. This air/fuel mixture can affect the fuel control system and can be seen in the fuel trim. If the activated carbon has no hydrocarbons adsorbed within it, the mixture will be lean. If the activated carbon has hydrocarbons contained within it, the mixture will be rich. If you have a fuel trim issue at idle and light load where the engine produces vacuum, disconnect and plug the purge valve. This will indicate if the fuel trim issue is caused by the purge control or not. 

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