1. Turn off the engine and wait until it cools down completely. Reach through the hole in the valve and press on its diaphragm with your finger; with moderate force the diaphragm should noticeably bend (refer to the accompanying illustration).
2. If the diaphragm does not bend or provides noticeable resistance, the EGR valve should be replaced. If you are not sure of the test results, compare the valve with a new one or one that is known to be in good condition.
A detailed description of the design and operating principle of the EGR system is given in Chapter Engine management and emission control systems.