A basic EGR system is simple, consisting of a vacuum operated EGR valve and a vacuum line from the carburetor. The EGR valve usually bolts to the engine intake manifold or a carburetor plate. Exhaust gases are routed through the cylinder head and intake manifold to the EGR valve. The EGR valve consists of a vacuum diaphragm, a spring, an exhaust gas valve, and a diaphragm housing. It is designed to control exhaust flow into the intake manifold. Though  there  are  minor  differences  between systems,  the  basic  operation  of  an  exhaust  gas recirculation system is as follows: At idle, the throttle plate in the carburetor or fuel injection  throttle  body  is  closed.  This  blocks  off  engine vacuum, so it cannot act on the EGR valve. The EGR spring holds the valve shut, and the exhaust gases do NOT enter the intake manifold. If the EGR valve were to open at idle, it could upset the air-fuel mixture and the engine would stall. When the throttle plate is swung open to increase speed, engine vacuum is applied to the EGR hose. Vacuum pulls the EGR diaphragm up. In turn, the diaphragm pulls the valve open. Engine exhaust can enter  the  intake  manifold  and  combustion  chambers.  At higher engine speeds, there is enough air flowing into the engine that the air-fuel mixture is not upset by the open EGR valve. There are two different methods of supply vacuum to the EGR valve as follows: The first method uses a vacuum port into the carburetor throat located just above the throttle plate. As the throttle begins to open, vacuum will begin to be applied to the port and operates the EGR valve. The valve  will  continue  to  operate  fully  until  approximately half throttle is reached.   As the throttle is open past the halfway point, exhaust gas recirculation gradually will diminish to zero, as the throttle approaches the fully opened position. The second method uses a vacuum port that is directly in the carburetor venturi (fig. 4-53). The carburetor venturi provides vacuum for the EGR valve any time the engine is running at high speed. The problem with using venturi vacuum is that it is not strong enough to open the EGR valve. So to make it work, manifold vacuum is used to operate the EGR valve  through  a  vacuum  amplifier.  The  vacuum amplifier switches the manifold vacuum supply to the EGR valve whenever venturi vacuum is applied to its signal port. At times of large engine loading (wide, open   throttle),   manifold   vacuum   will   be   weak, producing the desired condition of no exhaust gas recirculation. An engine coolant temperature switch may be used to prevent exhaust gas recirculation when the engine is cold. A cold engine does not have extremely high combustion temperatures and does not produce very much NO_x.    By blocking vacuum to the EGR valve below 100°F, you can improve the drivability and performance of the cold engine. FUEL EVAPORIZATION CONTROL SYSTEM The fuel evaporization control system prevents vapors from the fuel tank and carburetor from entering the   atmosphere   (fig.   4-54).   Older,   pre-emission vehicles used vented fuel tank caps. Carburetor bowls were also vented to the atmosphere. This caused a considerable  amount  of  emissions.  Modern  vehicles commonly use fuel evaporization control systems to prevent  this  source  of  pollution.  The  major  components of the fuel evapotization control systems are the sealed fuel tank cap, fuel air dome, liquid-vapor separator, roll- over valve, fuel tank vent line, charcoal canister, carburetor vent line, and the purge line. SEALED FUEL TANK CAP is used to keep fuel vapors from entering the atmosphere through the tank filler neck. It may contain pressure and vacuum valves that open in extreme cases of pressure or vacuum. When the fuel expands (from warming), tank pressure forces fuel vapors out a vent line or line at the top of the fuel tank, not out of the cap. FUEL AIR DOME is a hump designed into the top of the fuel tank to allow for fuel expansion. The dome normally provides about 10 percent air space to allow for fuel heating and volume increase. LIQUID-VAPOR SEPARATOR is frequently used to keep liquid fuel from entering the evaporation control system. It is simply a metal tank located above the main fuel tank. Liquid fuel condenses on the walls of the separator and then flows back into the fuel tank. ROLL-OVER VALVE is sometimes used in the vent line from the fuel tank. It keeps liquid fuel from entering the vent line after an accident where the vehicle rolled upside down. The valve contains a metal ball or plunger valve that blocks the vent line when the valve is turned  over. 4-46