Ok I'll try to sketch out the OEM tstat function although readers might want to look at pages 1 and 10 of kap's comments at
viewtopic.php?f=5&t=50940 for some good pictures and commentary on internal function. Keep in mind that our tstat functions upside down in the sense that it is the opening of the tstat that closes off the bypass port.
The tstat has 4 outflows and 1 inflow. The inflow is the rectangular hole in the head where the tstat bolts on and ALL coolant passing thru the tstat comes thru there from the engine block. Once coolant enters the body of the tstat the ports in the middle of the tstat body direct coolant flow, regardless of tstat position, thru 1) the large plastic port to the viscous heater and then to the heater core (before you ask yes per the FSM coolant circulates thru the heater core all the time) and 2) the small port that is both the coolant overflow and de-gas (constant air bleed) port to the plastic tank on the firewall. When the engine is cold and the tstat is closed (see the above upside down comment) a large portion of the coolant is directly circulated out the bottom aluminum port to the water pump inlet and then forced by the water pump thru the block and a small portion thru the large front facing port to the radiator. As the coolant going thru the water pump directly to the engine warms up the tstat opens to reduce and eventually close off the flow thru the bypass port so that all the flow now goes thru the 2 middle ports and the front facing port to the radiator.
As you can see if the tstat fails open or by opening too early, as ours typically does, coolant flow is directed away from the bypass port and to the radiator too early and is constantly over cooled hence the engine never fully warms up. In my experience during the summer a failing but not failed tstat can be masked by higher ambient temperatures and AC load.
On a cold engine an in line tstat will restrict the flow of coolant out that front facing port to the radiator until the in line tstat opens. This would have a tendency to increase the temperature within the block and tstat housing quicker at least until the bypass is closed or partially closed. However, as noted above with a properly functioning OEM tstat on a cold engine some flow is already directed thru that front facing port therefore IMHO if you are going to use an in line tstat it's important to do two things: 1) provide a flow path thru the skirt of the in line tstat to mimic that OEM flow, ensure the inline warms up from that flow, and to prevent temperature spikes and 2) to place the temperature sensitive portion of the in line tstat as close as physically possible to the OEM front facing port and make sure it stays there. The obvious potential problem with an in line is the balance between flow provided to the bypass port and flow to the radiator as too little flow to the radiator too late in the warm up cycle could result in overheating/temp spikes. Keeping in mind that the temp gauge sensor is on the OEM tstat housing I'd think excessive temperature should be virtually instantly visible and I'd also venture that if you experience warm-up temps going more than 1 tick above vertical that the hole in the skirt of your in line needs to be larger and I'd speculate that those using an in line in really hot areas like Tempe, AZ might need a bigger hole in the tstat skirt.
As for me and stoutdog; I'm running a replacement OEM pending receipt one of Kap's upgrades and stoutdog is running a Kap upgrade based on my old failed opening early core.
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The one I just got has a big flange around the 'stat body and will not fit into the hose. ???? ANyone else have this problem?
