Evaporative emission systems (EVAP) have been used as an anti-pollution system on most 1970 light-duty vehicles sold in California. Some medium/heavy-duty and 4x4 vehicles had later deadlines.  Starting in 1971, all Federal/USEPA vehicles (forty-nine states) were required to be equipped with some evaporative emission system. EVAP regulations prohibited uncontrolled hydrocarbons from entering the atmosphere from the evaporative emission system.    

Evaporative emission systems trap, store, and release gasoline vapors into the engine to mix with the incoming air/fuel mixture.  These gasoline vapors are also called volatile organic compounds (VOCs). Volatile organic compounds are those parts of a fuel’s composition that contribute to photochemical smog when introduced into the atmosphere. 

Over 20% of the hydrocarbons a motor vehicle generates result from fuel evaporation when the engine is not running.  Hydrocarbon emissions caused by evaporative emissions can be defined as follows: 

1.    Diurnal Loss: Vapors vent to the charcoal canister as the ambient temperature rises during the day.  Excess vapors will enter the atmosphere if the charcoal canister reaches a saturation point.  Diurnal loss is measured with the engine off and usually occurs within 35 minutes of shutdown. 

2.    Hot Soak: The engine remains hot long after the vehicle has been turned off.  For 35 minutes, emissions are measured to gauge this event.  Evaporation occurs from fuel that has been heated in the engine compartment and all fuel supply and vent lines.   

3.    Resting Loss: Gasoline permeates rubber and plastic components in the fuel system while the vehicle is at rest.  This event occurs when the vehicle has been turned off and allowed to sit for at least thirty-five minutes in a steady or decreasing ambient temperature condition. 

4.    Running Loss: Gasoline vaporizes as the engine is running.  The running loss test is conducted in an enclosure on a dynamometer.  The exhaust gases are routed out of the enclosure, leaving only vapors from evaporation.  The running loss test occurs over one hour while the vehicle is driven through three consecutive Federal Test Procedure (FTP) drive cycles.           

The EPA has devised the Sealed Housing Evaporative Determination Test (SHED).  All evaporative hydrocarbon emissions from fuel, tires, and plastics are collected and measured during this test.  This test includes a multi-day Diurnal Test (Enhanced Evaporative Emission Test), where the vehicle is repeatedly heated and cooled to simulate the effects of hot and cold ambient temperatures.  The figure below shows a drawing of the SHED setup. Courtesy of Colorado State University-NCVECS           

Factory emission tests have determined that an EVAP system with a leak as small as .020 inch can yield an average of 1.35 grams of hydrocarbons per vehicle per driven mile.  This is over thirty times the current allowable exhaust emissions standard. Evaporative System Operation  The Evaporative Emission Control (EVAP) system is designed to store and dispose of fuel vapors usually created in the fuel system, preventing its escape to the atmosphere.  The EVAP system delivers these vapors to the intake manifold to be burned with the usual air/fuel mixture.  This fuel charge is added during periods of closed-loop operation when the closed-loop fuel control system can manage the additional enrichment.  Improper operation of the EVAP system may cause rich driveability problems and failure of the Two-Speed-Idle test or Enhanced I/M loaded mode ASM test. The EVAP system is a fully closed system designed to maintain stable fuel tank pressure without allowing fuel vapors to escape into the atmosphere.  Fuel vapor is usually created in the fuel tank due to evaporation.  It is then transferred to the EVAP system charcoal canister when tank vapor pressures become excessive.  The canister absorbs fuel vapors until it becomes fully saturated.  When engine operating conditions can tolerate additional enrichment, these stored fuel vapors are purged into the intake manifold and added to the incoming air/fuel mixture. 

Types of Evaporative Emission Systems        

Identifying the type of EVAP system equipped on the vehicle is essential.  Since 1970, several methods have been used.  The following are the most common: 

1.    Non-ECM-controlled EVAP systems use solely mechanical means to collect and purge stored fuel vapors.  Typically, these systems use a ported vacuum purge port and a Thermo or Ported Vacuum Switch to prohibit system operation during a cold start or warm-up. 

2.  Pre-OBD II ECM Controlled EVAP systems use a ported or intake manifold vacuum purge source with a duty-cycled vacuum switching solenoid.  This type of EVAP system can provide more precise control of purge-flow volume and operation inhibition.  It cannot check the system for leaks.  Sometimes, the ECM does not know if purge flow occurred. 

3.    OBD II systems will be seen in either Non-Enhanced or Enhanced form.  This book will address these two systems: 

A)  Non-enhanced systems can only detect purge flow but cannot check the system for leaks.  These systems do not include a readiness monitor. They were installed on some 1996 and 1997 model-year vehicles.  

B)   Enhanced systems can detect purge flow and system leaks by using vacuum switches or pressure sensors.  A vent solenoid, vent valve, or closed canister valve can identify them.  Some systems test when the key is turned off.  All systems include a readiness monitor. Enhanced EVAP has required equipment on all 1998 and newer model-year vehicles. Enhanced EVAP systems can be divided into various types that accomplish the same goal using different methods.  Depending on the model year, enhanced evaporative emission systems must be capable of identifying leaks as small as .040 inch or .020 inch.  These systems perform the test by either lowering the pressure, thus creating a vacuum, or increasing the pressure through a pump. 

Methods of Testing Enhanced Evaporative Emission Systems 

1.    Vacuum Decay:  The vacuum method utilizes a purge solenoid, vent solenoid, or close canister valve (CCV) and a fuel tank pressure sensor (FTP).  These components monitor the evaporative emission system for leaks.  This system uses an engine vacuum through the purge solenoid to create a vacuum in the closed system.  This is the most common system in use today. 

2.    Fuel Tank Pressure/Canister Vacuum: This system has been used by Honda and Toyota and can isolate the fuel tank from the charcoal canister to pinpoint the leak.  Toyota utilizes two versions of this system: a non-intrusive system, which was used from 1996-2002, the second is called an intrusive system, which was introduced in 2000.  

3.    Leak Detection Pump: This system pressurizes or creates a vacuum in the evaporative emission system through a leak detection pump (LDP).  LDP has been used by Audi, BMW, Chrysler, Hyundai, Mitsubishi (Chrysler products), Volkswagen, and Volvo.  In 2000, BMW, Hyundai, and Mazda introduced a Diagnostic Module for Tank Leakage (DM-TL) pump to pressurize the system with the engine off.  Toyota 2005 introduced a vacuum pump module key-off system. 

4.    Engine off Natural Vacuum Leak:  This system works on the fluid’s vacuum/pressure principle in a closed container.  As temperature increases - the pressure increases if no leaks are present.  This system primarily tests for a very small leak (.020”).  It first appeared in 2002 model year vehicles, and Chrysler, Ford, GM, and Honda use variations of this system. To inspect or repair these systems, the technician must be able to identify them.