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.
"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.
Evaporative Emission Control Systems
The EEC system is a method of controlling HC emissions by collecting fuel vapours from the fuel tank and carburettor fuel bowl vents, and directing them into an engine intake manifold. The first evaporative emission control (EEC) was introduced on 1970 cars due to California’s stringent emission law, and was used on all old and 1971 cars. A typical evaporative emission control system is illustrated in Fig. 17.19. 17.6.1.
Evaporative Emission Control System (EVAP) Diagnostics
The evaporative emission control system prevents the uncontrolled release of gasoline vapors (hydrocarbons) into the atmosphere. These vapors are produced when fuel evaporates in the sealed fuel tank..