The EGR valve is responsible for recirculating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture. Nitrogen Oxide emissions are formed when engine chamber temperatures rise
above 2500* F. Keeping the chamber cool reduces NOx emission.
The higher the combustion temperatures, the greater the formation of NOx. Along with the CAT, the EGR valve significantly reduces unburned hydrocarbons HC and Nitrogen Oxide NOx in exhaust emissions.
EGR valves can be both vacuum controlled or electronic. In both cases they are normally located near the intake manifold around the top of the engine.
During the Smog Check: During the Basic Smog Test the smog machine is not designed to actually look at the EGR System through the engine's exhaust.
Rather, it has the smog technician visual and functionally inspect the EGR system through the use of a vacuum pump with pressure gauge, and then has him or her enter this information into the smog machine. So actually, the smog machine has no knowledge of the EGR system failure other then the technician's input.
If the EGR system or test has failed or fails during the emissions inspection, the technician will enter this information manually into the smog machine.
Now as for the Enhanced Smog Test, the smog test machine reads NOx emissions through the tailpipe which eliminates the need for the smog check technician to inspect the EGR system physically. During the Enhanced Smog Test the smog machine determines whether or not the EGR system is functioning properly based on the NOx emissions content. NOx is always created when there is an EGR malfunction either due to a plugged valve or defective vacuum system.
The EGR Valve Can Get Plugged Up: Since the EGR valve simply pulls exhaust gasses from your exhaust and introduces them back into your engine's intake air over time it will become clogged or stuck open/closed from the carbon deposits. It is possible to clean the EGR valve if you remove it from your vehicle and either spray it out or let it sit in regular gasoline. This cleaning process can not be used for an Electronic EGR valve without removing the electrical components first.
How To Test The EGR Valve: This test only applies to vacuum controlled EGR valves only. A defective EGR valve will not open or close when vacuum is applied. The next step is to find out whether the EGR passages are plugged with carbon deposits or if the passages are open.
The way to test this out is to start your vehicle and let it run at idle. Disconnect the EGR valve's vacuum line and apply direct vacuum in order to open the valve. When the valve opens; if the passages are clean and open you should notice a large RPM decrease. If the passages are plugged up, you will not see any RPM difference. If there is a RPM decrease, then the passages are free, and the next step is to inspect the vacuum signal which is supposed to open the EGR valve. It may be possible your vehicle is not opening the EGR valve by applying vacuum at the right time, or at all. If there is no RPM decrease, the passages may be plugged up. You will need to disable the EGR valve and clean all passages within the valve and within your engine's intake and exhaust passages eliminating any obstructions.