4.2L
With engine not running, disconnect vacuum hose from EGR valve. Connect a hand-held vacuum pump to EGR valve. Apply vacuum to EGR valve and ensure that diaphragm will hold vacuum until released. EGR valve should close when vacuum is released. Start engine. Disconnect vacuum hose to EGR valve and plug. Apply vacuum to EGR valve. Engine should run rough or stall. If system operates as indicated, system is functioning properly.
5.3L
Testing procedure is not available. The 5.3L engine is equipped with 2 EGR valve solenoids (one on each side of engine, mounted to intake manifold below air cleaner housing). EGR solenoid valves are electronically controlled. There is no exhaust gas recirculation at idle or full throttle. EGR valves will not function until engine temperature reaches 95°F (35°C), and on some models, vehicle speed must be greater than 60 MPH.