Coolant Flow with Hoses Looped (or with a heater core)
On a cold start with the thermostat closed, almost all of the water exits the engine through the port on the rear of the cylinder head and goes to the passenger side heater hose tube. The coolant passes by the temperature sensor at this point. The water exits the first heater hose tube and either goes to our hose loop or to a heater core. It then returns through the driver side heater hose tube. The hot water from this tube strikes the back side of the thermostat and begins to heat it up.
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When the water striking the thermostat is hot enough, the thermostat opens and water begins to flow from the passenger end of the cylinder head, up to the radiator and then back to the thermostat. Even after the thermostat is open, you still get coolant flow from the driver end of the cylinder head through the heater hoses.
Coolant Flow with the Heater Tubes Capped/Plugged
If the heater tubes are blocked off, the only flow you have early on is through the small bypass holes in the thermostat. Instead of getting hot water hitting the thermostat early on, you will have a small flow of cool water passing by. The thermostat will stay closed much longer than before because the only water it is seeing is the cool water. The hot water from the engine will slowly work its way to the front, through the radiator and back to the thermostat. The flow of this hot water will be restricted by the thermostat. There is no telling how hot your engine will get before the thermostat finally opens. Perhaps heat will find its way to the thermostat from the cylinder head heating the thermostat housing but you will still have a large supply of cool water from the radiator slowly passing by the thermostat to cool it off.
You would never have water flowing past the engine coolant temperature sensor so your readings won't be as accurate.
Finally, it is possible the engineers that designed the passages inside the block and head took into account that water would be exiting near the thermostat housing. Without that opening, the water may not flow as they intended and you could end up with a stagnate area on the driver side of the block and head.
We recommend looping the hoses to maintain the coolant flow as the engineers originally intended it to be.