Standard Cooling System (CTO) Valve

The distributor vacuum advance is controlled by carburetor ported vacuum after engine coolant warms to a predetermined temperature. Warm-up driveability is improved by controlling vacuum advance with manifold vacuum while engine is cold. The CTO valve is screwed into the intake manifold coolant passage on 4.2L engines, and into the thermostat housing or intake manifold coolant passage on 5.9L engines. The CTO valve monitors engine coolant temperature at all times. Depending on coolant temperature, CTO will permit either manifold vacuum, or carburetor ported vacuum to control distributor vacuum advance.