What the EGR valve does is channel a measured amount of exhaust gas back into the combustion chamber to lower combustion temperatures. Like hosing down a fire. What this accomplishes is to lower Nitrogen Oxide (NOx) emissions and can often increase fuel mileage.

As the throttle is applied and the engine speeds up, ported vacuum is applied to a vacuum diaphragm in the EGR valve through a connecting tube. When the vacuum reaches approximately three inches, the diaphragm moves upward against spring tension and is in the full-up position at approximately seven to eight inches of vacuum.

This diaphragm is connected by a shaft to a valve that closes off the exhaust gas port. As the diaphragm moves up, it opens the valve in the exhaust gas port and allows exhaust gas to be pulled into the intake manifold and enter the cylinders. The exhaust gas port must be closed during idle, as the mixing of exhaust gases with the fuel air mixture at this point would cause rough running.

Vehicles with an OBD-II diagnostic capable PCM can monitor the EGR valve for malfunctions and proper exhaust flow. When it detects a malfunction, it will turn the MIL on since it is directly related to an emission problem.

There are a couple of ways to quickly test an EGR valve. One is to simply unplug it, cap off the vacuum line and drive the vehicle. If the problem goes away, you have a problem with the EGR control system.

The other way is to connect a hand vacuum pump to the EGR valve and slowly apply vacuum to it. If the engine rpm slows down and stumble and probably stall when you get to full vacuum.

If there is no change in the engine rpm remove the EGR valve and perform the same test, this time watching the valve to open. If it does, then you have clogged EGR passages.

If the engine has been idling rough, look for heavy carbon deposits around the pin valve. If there are any clean them off with a wire brush. This will only work with an exposed pin EGR valve. If yours is a recessed pin EGR valve, it will have to be replaced.