This is the drivers side "Y" split that comes out of the thermostat housing/radiator hose. The heater hose that taps off of this goes to the shut off valve on the drivers side.


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You will find a smaller "Y" somewhere on the drivers side, downstream from the shut-off switch. One side of the smaller Y connects to the other small Y on the RH side. This allows water to circulate when one or both valves are turned off. If you didn't have this feature, you could potentially get cavitation in the water pump as air bubbles collect at the top of the impeller with no way to get out.

Even Mustangs and Station Wagons have this provision. Sometimes in the form of an H-pipe somewhere in the two heater lines, OR, a small hole in the shut-off valve that still allows some water movement.

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