Originally Posted by bigmac
The heater inlet hose is usually plumbed to the intake manifold, and the heater outlet hose returns to the circulating pump. There are usually two plumbed fittings on the intake manifold, one of which has the water temp sensor, and the other one is plugged unless you have a heater. If you have a heater, there's often a valve there to close down heater circulation (doesn't work very well, IMHO). Likewise the heater return port on the circulating pump is only plumbed if there's a heater. Some people (and some mfgrs) plumb the heater intake line directly to the raw water intake using a Y fitting, the point of which (AFAIK) is to allow more heat to be extracted by the heater core. In that case the heater return line is connected to the barb on the Y fitting, and the usual return port on the circulating pump is plugged.
so you're saying that because I don't have a heater, i may not have this 'line' running back to my recirc pump.. So I guess, if my thermostat is closed, the thermostat can route the water that just circulated through the engine back to the hose that leads to the recirc pump, and as the thermostat opens fully, it can somehow block the just circulated water from returning back to the pump (forcing it to exit out the exhaust) and allow only raw water to be drawn into the pump... I'd like to see the plumbing on the inside of the thermostat housing to see how this takes place.