The exhaust gas recirculation valve (EGR valve) recirculates combustion gases—including unburned hydrocarbons—back into the engine intake. This has the desirable effect of lowering the peak flame temperature in the combustion process, thus reducing the production of oxides of nitrogen, a major pollutant.
If the EGR valve does not open and close properly, the Check Engine Light will illuminate and a fault will be stored. Engine performance problems (e.g. poor idle quality or "pinging" under acceleration) can also occur.
To replace the EGR valve, the mechanic removes it from the intake manifold and then checks the EGR passageways in the EGR system to ensure they are clear; then, the new EGR valve is installed.
Note: Access to the EGR valve can be extremely limited on some vehicles.
Mechanics' Corner: More Technical Detail
- The exhaust gas recirculation (EGR) system recycles a small amount (usually no more than 10 percent) of exhaust gas from the exhaust system and mixes it with the intake manifold air going into the combustion chambers. Adding this inert (noncombustible) exhaust gas limits the peak combustion temperatures to less than 2,500° F (when the formation of NOx is known to occur). The flow of this recycled exhaust gas is managed by either a vacuum or an electrically-controlled, pintle-shaped EGR valve. There are two ways the valve most commonly fails. First, it may stick in the open position, which causes a very rough idle (and a lack of power). The EGR valve can also fail in the closed position, which causes detonation on acceleration; if this happens, the vehicle will fail an emissions test for high NOx. In many cases, but not always, a Check Engine Light will illuminate to alert the driver of this failure.
- To replace the EGR valve, first remove it from the intake manifold, which is usually near the throttle body. To make sure the new EGR valve doesn't fail, clean all the EGR passages so the system will function properly. Always have a skilled emissions diagnostician check the EGR system before and after the repair to verify that all its other components work properly. It's always best to use factory-original EGR valves because they are tightly calibrated to match each engine configuration. Make sure that any engine codes are cleared and perform the proper drive cycle to make sure the emissions computer clears the self-testing of the new EGR valve without any errors.