Evaporative Emission Control Systems

P0444 Evaporative Emission Control System Purge Control Valve Circuit Open

Dorman 911-632 Evaporative Emissions Charcoal Canister

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  • TAG : P0447 Evaporative Emission Control System Vent Control Circuit Open
  • The evaporative emission control system is designed to prevent fuel tank and carburetor bowl (if equipped) vapors from being emitted into the atmosphere. Fuel vapors are absorbed and stored by a fuel vapor charcoal canister. The canister stores them until certain engine conditions are met and the vapors can be purged and burned by the engine.

    The code P0446 is set when the engine computer recognizes a fault or restriction at the vent control part of the Evaporative emission control system (EVAP). The description of the fault varies between different car manufacturers, but one thing is common, the code P0446 doesn't directly point to a defective part; some testing might be required to pinpoint the faulty component. Read how the EVAP system works below.

  • 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 code P0455 is set when the engine computer recognizes a large leak in the Evaporative emission control system (EVAP). The vehicle's EVAP system is sealed, it's main purpose is to prevent gasoline vapors in the fuel tank from escaping into the atmosphere. The most common culprit is the gas cap that is left open or not closed properly, although there could be other problems, read more below.

    Evaporative Emission Control System 1988-1991

  • Evaporative Emission Control System 1988-1991

    The EVAP system is a fully closed system designed to maintain stable fuel tank pressures without allowing fuel vapors to escape into the atmosphere. Fuel vapor is normally created in the fuel tank as a result of evaporation. It is then transferred to the EVAP system charcoal canister when tank vapor pressures become excessive. When operating conditions can tolerate additional enrichment, these stored fuel vapors are purged into the intake manifold and added to the incoming air/fuel mixture. Video EVAP Evaporative Emission Control Systems This is necessary in modern vehicles with plastic fuel tanks to allow for expansion and contraction and to prevent vacuum lock. The charcoal canister is located under the hood or under the car and is responsible for absorbing the hydrocarbons or volatile organic compounds (VOC's) contained in fuel vapors. The vent solenoid is normally open to allow outside air to flow through the charcoal and release the vapors from the charcoal when the purge solenoid is open. Most PCM's only allow purging during closed-loop operation at cruising speeds.

"How Evaporative Emission Control Systems Work" 23 February 2012.

The fuel tank filler caps used on cars with EEC sys­tems differ from those used on cars without EEC systems. Most caps in EEC system are incorporated with built in pressure-vacuum relief (Fig. 17.20), so that a vacuum lock may develop due to the fuel expansion or contraction. Fuel tanks are protected in different ways against fuel expan­sion and overflow, caused by heat. An overfill limiter, or temperature expansion tank, was fitted on many 1970-73 EEC systems to limit total filling of the tank. This is installed inside the fuel tank and has small holes, which open it to the fuel area. When the fuel tank appears to be completely full, it holds no more and the fuel gauge reads full although the expansion tank remains virtually empty. This offers enough space for the expansion of fuel and
collection of vapour if the vehicle is parked in the hot sun after filling the tank.

Fig. 17.18. PCV valve operation in case of backfire.

Fig. 17.19. Evaporative emission control system
The dome shape of the upper portion of the fuel tank incorporated in some later-model cars, or the overfill limit­ing valve installed inside the vapour-liquid separator, eliminates the need for the overfill limiter tank fitted in earlier systems.
Some Ford-built cars use a combination valve, which performs the following tasks :
(i) It isolates the fuel tank from engine pressures and permits vapour to escape from the vapour separator tank to the vapour storage canister.

Fig 17.20. EEC systems fuel tank caps.
(ii) It vents excess fuel tank pressure to the atmosphere in case the vapour delivery line is blocked.
(Hi) It allows fresh air to be drawn into the fuel tank to fill the space created by petrol as it is used.
All EEC systems incorporate some type of liquid-vapour separator to prevent liquid fuel from reaching the engine crankcase or vapour storage canister. Some liquid-vapour separators are con­tained within the tank and use a single vapour vent line from the tank to the vapour canister. When the separator is not built into the tank (Fig. 17.21) it is usually installed on the outside of the tank or on the frame near it. In this case, vent lines extend from the tank to the separator and are arranged to vent the tank, irrespective of whether the car is on a level surface or not. Liquid fuel entering the separator returns to the tank through the shortest line.

17.21. Liquid-vapour separator is mounted separately from the tank.