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Source is pressure side before the thermostat so coolant will flow back to the pump when the thermostat is closed. This for a Chevy is on the intake manifold. The typical places Chevy has done the source is either off the backside of the intake with a tap into the unused coolant return on the back of the head or from a fitting on the front of the intake into one return passage leading to the thermostat collector under the water neck return to the radiator.

The return side is to the inlet side of the pump. Chevy has about 3 configurations of this the oldest being a fitting on the top of the pump which connects the return hose to a passage inside the pump to the inlet cavity ahead of the impeller. In later vehicles one can find a return hose leading to a fitting on the cool side tank of the radiator which feeds the pump intake. Another version of this has a Tee or Y fitting in the lower pump intake hose from the radiator.

Any of these configurations satisfies the source being on the pressure side and the return being on the suction side.

If you have air conditioning there will be a three way, vacuum operated valve in the bypass hoses going to and from the heater. This valve functions to redirect the bypass coolant such that the heater is shut off but the closed thermostat bypass function is maintained. For A/C models this is done to eliminate the need for the cooled air to overcome coolant heated air as well as hot cabin air that the outside environment is providing.

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