How the Evap System Works ?

Beautiful Girls Doing Auto Repairs
How the Evap System Works ?
How the Evap System Works ?

In this Video Lovely Jade explains how the Evaporative Emission Control System Works, also known as the EVAP System in a way that is simple to Understand wearing a Sexy Curve Hugging Dress and Heels.

How the Evap System Works ?

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


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.

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