There are several types of EGR Valve, earlier systems use a vacuum-operated valve, whilst newer vehicles are electronically controlled.
Diesel High Pressured EGR Valves:
Divert The high-flow, high-soot exhaust gas before it enters the diesel particulate filter – the soot can combine with the oil vapor to create sludge. The gas is then passed back to the inlet manifold either via a pipe or internal drillings in the cylinder head. A secondary valve is also used to help create a vacuum in the inlet manifold as this is not naturally present on diesel engines.
Diesel Low Pressure EGR Valves:
Divert the exhaust gas after it has passed through the DPF. This gas has a lower flow but is almost completely clean of soot. The gas is then passed back to the inlet manifold via a pipe.
Gasoline EGR Valves:
Divert the exhaust gases, much like the high pressure diesel equivalent. The vacuum created by the cylinder depression, draws the exhaust gases in and the flow is regulated by the opening and closing of the EGR valve itself.
Digital EGR Valves:
Feature a solenoid or stepper motor and in most cases, a feedback sensor. These valves receive a pulse width modulated signal from the ECU, to regulate exhaust gas flow.