The Exhaust Gas Recirculation system was introduced to help reduce harmful exhaust emissions. The main ones being Nitrogen Oxides or (NOx) NOx is the combination of Nitrogen and Oxygen during combustion. The EGR valve recirculates exhaust gas from the exhaust manifold into the inlet manifold in a controlled manner. Since the gasses that are being passes into the inlet manifold have been previously burned, they can not be burned again so they act as to dilute and lower the amount of oxygen in the air intake. This lowers the combustion temperature and in turn lowers the NOx emissions produced. 

• The EGR Valve can be an electronic solenoid directly controlled by the Ecm or a diaphragm operated by an electronic vacuum regulator which in turn is operated by the Ecm.

   

• The Gasses are passed from the Exhaust manifold to the Inlet manifold via a tube, this is made of Steel because of the high temperature's involved. The Ecu is programed to open the Egr at predefined points. The Ecu will make the choice to open the Egr by monitoring such things as Engine speed, throttle position, manifold pressure and engine temperature. When the right conditions are met , the Egr will open and allow a measured amount of exhaust gas to pass into the Inlet manifold.

   

• The Egr flow is measured in various ways depending on the vehicle you have.  Manifold pressure, Engine temperature, Air fuel Ratio (Measured by The lambda sensor) or Pressure (using a DPFE Sensor - Delta Pressure Feed Back)

Testing with a Voltmeter:  If you have an electronic type then it is more difficult to test and fault codes and serial data will be needed. If you have the 2 pin solenoid type with a vacuum connection to a diaphragm  then you will find 12 volts on both terminals. One terminal is the 12 volt supply and the other is the switch side. The Ecm will switch the 12 volts to earth to activate the valve.

The Solenoid will only be switched when the right engine conditions are met. It has to be warm and often 3000 rpm or above.

The operation of the solenoid can also be checked with a vacuum Gauge. The solenoid electrics can often give a good reading yet the business end of the solenoid does not allow vacuum to act on the Diaphragm.

Testing with an Oscilloscope:
 

Readings taken with a Pico Automotive Oscilloscope.
 

The image above shows the solenoid voltage being switched to earth. At this point the Egr is open. This reading should also be backed up by using a vacuum gauge, You are best to connect the gauge into the Vacuum circuit using a T piece. This prevents the circuit being interrupted. If the pipe was removed then the Ecm may issue a fault code and put the system into limp mode preventing the Egr from operating.