C 20 3/4 Ton Pickup 2WD V8-350 5.7L VIN L 4-bbl (1983)
Thermal Vacuum Valve - EVAP: Testing and Inspection
EGR Canister Purge Thermal Vacuum Switch (EGR-CP-TVS)
Below the switching point, Ports 1 and 2 should be blocked and no ported vacuum should be directed to the canister and EGR valve.
Above the switching point, Ports 1 and 2 should be open, allowing ported vacuum to be directed to the canister and EGR valve. Replace valve, if
defective.