EGR System (420 & 560 Series Only)

Apply about 12 in. Hg to EGR valve. Disconnect vacuum line. The EGR valve should close. If not okay, replace EGR valve. If okay, go to next step.
Run engine at idle speed. Apply about 12 in. Hg to EGR valve. Engine should run rough at idle. If not okay, replace EGR valve. If okay, go to next step.
Connect vacuum tester to EGR vacuum line using a "Y" fitting. Increase engine speed slowly to about 4000 RPM. Tester should indicate vacuum. If okay, test is complete. If not okay, go to next step.
Disconnect vacuum line from thermo valve and connect vacuum tester with "Y" fitting to vacuum line and thermo valve and apply vacuum. Thermo valve should be open with coolant temperature above 122°F (50°C) and closed with coolant temperature below that. If okay, proceed to next step. If not okay, replace thermo valve.
Test thermo valve for leaks. Seal straight connection of thermo valve. Check and ensure vacuum is held at angular connection to thermo valve. If okay, test is complete. If not okay, replace thermo valve.