evaporative emission control system
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EVAP Evaporative Emission Control System

by Larry Carley copyright AA1Car.com

The Evaporative Emission Control System (EVAP) is used to prevent gasoline vapors from escaping into the atmosphere from the fuel tank and fuel system.

The EVAP system usually requires no maintenance, but faults can turn on the Check Engine light and prevent a vehicle from passing an OBD II plug-in emissions test.

The OBD II EVAP monitor on 1996 and newer vehicles runs diagnostic self-checks to detect fuel vapor leaks, and if it finds any (including a loose or missing gas cap), it will set a fault code and turn on the Check Engine light. However, the EVAP monitor only runs under certain operating conditions. This may create a problem for the vehicle owner if his vehicle must be given an OBD II plug-in emissions test and the monitor has not completed.

EVAP Evaporative Emission Control system

Common problems with the EVAP system include faults with the purge valve that vents fuel vapors to the engine, leaks in vent and vacuum hoses, and loose, ill-fitting or missing gas caps. The most common fault code is P0440, which indicates a large leak (often a loose gas cap). EVAP Purge valve codes (P0443 to P0449) are also common).

The code you don't want to see is a P0442. This indicates the system has detected a SMALL leak, but small leaks can often be a BIG problem to find. By small, we mean a leak no larger than a pin prick! Such small leaks are virtually impossible to find visually, so a special tester called a smoke machine is usually necessary to reveal the leak. The smoke machine feeds a mineral-oil based vapor into the EVAP system under light pressure (no more than a few pounds per square inch). The smoke may also contain an ultraviolet dye to make it easier to see under UV light.

Fixing EVAP codes can be a challenge, even for professional technicians. And if you have a P0442 small leak code, you will probably have to take your car to a repair shop that has a smoke machine.


The EPA requires EVAP systems on cars because gasoline fuel vapors contain a variety of different hydrocarbons (HC). The lighter elements in gasoline evaporate easily, especially in warm weather. These include aldehydes, aromatics, olefins, and higher paraffins. These substances react with air and sunlight (called a photochemical reaction) to form smog. Aldehydes are

often called instant smog because they can form smog without undergoing photochemical changes.

The bad thing about fuel vapors is that fuel evaporates any time there is fuel in the tank. That means if the fuel system is unsealed or open to the atmosphere, it can pollute 24 hours a day even if the vehicle is not being driven. Uncontrolled evaporative emissions like this can account for as much as 20 percent of the pollution produced by a motor vehicle.

The EVAP system totally eliminates fuel vapors as a source of air pollution by sealing off the fuel system from the atmosphere. Vent lines from the fuel tank and carburetor bowl route vapors to the EVAP storage canister, where they are trapped and stored until the engine is started. When the engine is warm and the vehicle is going down the road, the PCM then opens a purge valve allowing the vapors to be siphoned from the storage canister into the intake manifold. The fuel vapors are hen burned in the engine.

Evaporative emission controls were first required on cars sold in California in 1970. EVAP has been used on all cars and light trucks since the early 1970s.


Sealing the fuel tank is not as simple as it sounds. For one thing, a fuel tank must have some type of venting so air can enter to replace fuel as the fuel is sucked up the fuel pump and sent to the engine. If the tank were sealed tight, the fuel pump would soon create enough negative suction pressure inside the tank to collapse the tank. On older EVAP systems, the tank is vented by a spring-loaded valve inside the gas cap. On newer vehicles, it is vented through the EVAP canister.


The major components of the evaporative emission control system include:

  • Fuel tank, which has some expansion space at the top so fuel can expand on a hot day without overflowing or forcing the EVAP system to leak.
  • Gas cap, which usually contains some type of pressure/vacuum relief valve for venting on older vehicles (pre-OBD II), but is sealed completely (no vents) on newer vehicles (1996 & newer). NOTE: If you are replacing a gas cap, it MUST be the same type as the original (vented or nonvented).
  • Liquid-Vapor Separator, located on top of the fuel tank or part of the expansion oerflow tank. This device prevents liquid gasoline from entering the vent line to the EVAP canister. You do not want liquid gasoline going directly to the EVAP canister because it would quickly overload the canister's ability to store fuel vapors. The liquid-vapor separator is relatively trouble-free. The only problems that can develop are if the liquid return becomes plugged with debris such as rust or scale from inside the fuel tank; if the main vent line becomes blocked or crimped; or if a vent line develops an external leak due to rust, corrosion, or metal fatigue from vibration.

    Some liquid-vapor separators use a slightly different approach to keeping liquid fuel out of the canister vent line. A float and needle assembly is mounted inside the separator. If liquid enters the unit, the float rises and seats the needle valve to close the tank vent.

    Another approach sometimes used is a foam-filled dome in the top of the fuel tank. Vapor will pass through the foam but liquid will cling to the foam and drip.

    If a blockage occurs in the liquid-vapor separator or in the vent line between it and the EVAP canister, the fuel tank will not be able to breathe properly. Symptoms include fuel starvation or a collapsed fuel tank on vehicles with solid-type gas caps. If you notice a whoosh of pressure in or out of the tank when the gas gap is removed, suspect poor venting. You can check tank venting by removing the gas cap and then disconnecting the gas tank vent line from the EVAP canister. If the system is free and clear, you should be able to blow through the vent line into the fuel tank. Blowing with compressed air can sometimes free a blockage. If not, you will have to inspect the vent line and possibly remove the fuel tank to diagnose the problem.

  • EVAP Canister. This is a small round or rectangular plastic or steel container mounted somewhere in the vehicle. It is usually hidden from view and may be located in a corner of the engine compartment or inside a rear quarter panel. The canister is filled with about a pound or two of activated charcoal. The charcoal acts like a sponge and absorbs and stores fuel vapors. The vapors are stored in the canister until the engine is started, is warm and is being driven. The PCM then opens the canister purge valve, which allows intake vacuum to siphon the fuel vapors into the engine. The charcoal canister is connected to the fuel tank via the tank vent line. Under normal circumstances, the EVAP canister causes few problems. Since the charcoal does not wear out, it should last the life of the vehicle.

    The most common problem with the EVAP canister is a faulty purge control or vent solenoid. Vacuum-type purge valves can be tested by applying vacuum directly to the purge valve with a hand-held vacuum pump. The valve should open and not leak vacuum if it is good. With solenoid-type purge valves, voltage can applied directly to the solenoid to see if the valve opens. The resistance of the solenoid can also be checked with an ohmmeter to see if it is open or shorted.

    The purge control strategy on many late model EVAP systems can get rather complicated, so the best advice here is to look up the EVAP diagnostic procedures in the OEM service literature.

  • The OBD II EVAP monitor tests the fuel system for vapor leaks.
    OBD II EVAP evaporative emission control system schematic


    On 1996 and newer vehicles, the OBD2 system monitors the fuel system for fuel vapor leaks to make sure no hydrocarbons are escaping into the atmosphere. The EVAP monitor does two things: it verifies there is airflow from the EVAP canister to the engine, and that there are no leaks in the fuel tank, EVAP canister or fuel system vapor lines.

    The OBD2 EVAP monitor runs once per drive cycle and only when the fuel tank is 15 to 85% full. The EVAP monitor uses a "purge flow sensor" to detect leaks as small as .040 inches in diameter on 1996-99 models, and as small as .020 inches on most 2000 and newer vehicles.


    If the OBD EVAP monitor detects a leak when it runs the EVAP leak check, it will set a fault code in the P0440 to P0457 range:

    P0440....Evaporative Emission Control System Fault
    P0441....Evaporative Emission Control System Incorrect Purge Flow
    P0442....EVAP Emission Control System Leak Detected (small leak)
    P0443....EVAP Emission Control System Purge Control Valve Circuit
    P0444....EVAP Purge Control Valve Circuit Open
    P0445....EVAP Purge Control Valve Circuit Shorted
    P0446....Evaporative Emission Control System Vent Control Circuit
    P0447....EVAP Emission Control System Vent Control Circuit Open
    P0448....EVAP Emission Control System Vent Control Circuit Shorted
    P0449....EVAP Emission Control System Vent Valve/Solenoid Circuit
    P0450....Evaporative Emission Control System Pressure Sensor
    P0451....EVAP Emission Control System Pressure Sensor
    P0452....EVAP Emission Control System Pressure Sensor Low Input
    P0453....EVAP Emission Control System Pressure Sensor High input
    P0454....EVAP Emission Control System Pressure Sensor Intermittent
    P0455....EVAP Emission Control System Leak Detected (gross leak)
    P0456....EVAP Emission Control System Leak Detected (small leak)
    P0457....EVAP Emission Control System Leak Detected (fuel cap)

    If you find a P0440, P0455 or P0457 fault code (large fuel vapor leak), remove the gas cap, inspect the seal on the filler tank inlet and the underside of the gas cap for any nicks, debris or damage. Then screw the gas cap back on and make sure it clicks at least once to assure a tight seal. If a fuel vapor leak at the gas cap set the code, the fault should clear and the Check Engine light go out the next time the EVAP monitor runs. If the light stays on, the problem is either a bad gas cap or a large vapor leak somewhere in the EVAP system (most likely a leaky or loose vapor hose).

    Evaporative Emission Control System Leak Detection

    Finding leaks in the EVAP system can be very difficult. It often requires using a special "smoke machine" that generates a fine mineral oil mist that is pumped into the EVAP system under very light pressure. The mist circulates through the plumbing and eventually seeps out through the leak, making the leak visible. The mist may also contain ultraviolet dye to make any leaks more visible when illuminated with a UV lamp.

    EVAP System Reference Files:

    EVAP Diagnostics (pdf file courtesy AC Delco)

    Nissan EVAP Canister TSB (pdf file courtesy Nissan)

    Toyota EVAP System Basics (pdf file courtesy Toyota)

    book Related Articles:

    Gas Cap Loose?

    Understanding OBD II Driveability & Emissions Problems

    Fixing Emission Failures

    All About Onboard Diagnostics II (OBD II)

    Basic Emission Control Systems Overview

    Finding & Fixing Vacuum Leaks

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