EGR Valve Function
Excessive combustion chamber temperature creates oxides of nitrogen, which is a major pollutant. The purpose of the exhaust gas recirculation valve (EGR) valve is to meter a small amount of exhaust gas into the intake system, which dilutes the air/fuel mixture so as to lower the combustion chamber temperature. An EGR valve remains in the closed position when the engine is started, at idle, and at wide open throttle.
Although an EGR valve is the most effective method of controlling oxides of nitrogen, by design it adversely affects engine performance. Your classic car engine was not designed to run on exhaust gas!
To combat this unwanted situation, the amount of exhaust entering the intake system has to be carefully monitored and controlled. This is done through a series of electrical and/or vacuum switches, as well as the vehicle computer. Since EGR action reduces performance by diluting the air /fuel mixture, the system does not allow EGR action when the engine is cold or when the engine needs full power.
There are different types of EGR systems used on different engines. The vacuum operated EGR valve is the most common. The valve is opened by vacuum supplied through a duty cycle solenoid and closed by a spring. The solenoid controls the valve to open in proportion to throttle opening.
A faulty EGR valve or related component may leave the valve in either the stuck open or stuck closed position. The symptoms of a stuck-open EGR valve are a rough idle with a lean air fuel mixture, running as if there's a major vacuum leak. The symptoms of a stuck-closed EGR valve are an engine that has pre-ignition ping and knock. This is the result of high cylinder temperatures, caused by the lack of the cooling exhaust gas introduced into the chamber by the EGR.
Exhaust emissions are also reduced through your catalytic converter. In good working order, converters do not significantly restrict exhaust flow.