The EGR valve is usually placed on the intake manifold. Most recirculation system failures are due to sticking or damage to this valve.
1. Stop the engine and wait for it to cool completely. Reaching through the hole in the valve, press the diaphragm with your finger, with a moderate effort, the diaphragm should sag noticeably (refer to accompanying illustration).
2. If the diaphragm does not flex or shows noticeable resistance, the EGR valve should be replaced. If you are unsure of the test results, compare the valve with a new one or a known good one.
A detailed description of the design and principle of operation of the EGR system is given in Chapter Engine management and emission control systems.