Modern vehicles have what's called an exhaust gas recirculation (EGR) valve to improve their emissions efficiency. Diesel vehicles will have something similar as well.
How Does An EGR Valve Work?
The EGR valve is connected to the exhaust manifolds and allows some of the exhaust gasses to be recirculated into the valves to mix with the air and fuel and be re-burned during the combustion process. This more thoroughly burns the exhaust particulates and gases, making them cleaner prior to emission from the tailpipe. When properly maintained, an EGR system will cause only a nominal loss in engine performance with the gain of better emissions control.
Signs the EGR Valve is Not Working Properly
When the EGR valve is not functioning properly, it will cause problems that, with some exceptions, are hard to notice. If you live in a region that requires exhaust emissions testing as part of your vehicle's registration process, you may notice a dramatic rise in emissions numbers or even fail to pass inspection due to improper EGR valve function. If your engine begins running badly, knocking, or you lose fuel economy, then it may be a defective EGR valve.
Diagnosing a Bad EGR Valve
Most of the time, your vehicle's computer will notice a bad EGR and will give a code. Otherwise, here are warning signs to watch for and things to do in order to diagnose that it is the EGR causing the problem:
Vehicle does not accelerate well, chokes out at stop signs or lights, and idles very roughly
This is a good sign that your EGR valve is stuck open and is sending too much exhaust into the engine for reburn, causing the idle to be sporadic and the engine to choke at low idle. If your vehicle seems to run just fine at high speeds (highway), but has trouble idling and at lower speeds (in town), then the EGR may be to blame. Depending on the vehicle, you may need to replace or clean the valve to restore function.
Emissions testing shows too much (or a big increase in) particulate matter or NOx gases
This indicates that the valve is either stuck closed or is not receiving signals to open. Some tests will test both low and high RPMs and if the emissions are too high only at high RPMs, then it's a sure sign your EGR is not opening. It will likely require replacement, though the problem could also be the electronic control of the valve, so check breakers and circuits as well.
Sudden loss of fuel economy, but engine otherwise running fine
This is a sign that the EGR valve is working properly at some speeds, but there is a signaling problem between its controller and the valve itself. Often this is an electronic problem on newer engines, though old engines may use an EGR controlled by vacuum and so may have a leaking line.
Replacing the EGR
A competent home DIY mechanic can likely replace the EGR valve with little trouble. Some specialty tools and replacement gaskets may be required, however, so read your manual thoroughly before beginning.
Related emissions pages