A mid sized update: I did an oil and filter change (Kirkland Signature 15W-40 + STP Extended Life S8AXL), fuel filter change, and thermostat (Aisin THT007) + top ring gasket (90430-43002) + housing gasket (16341-61030) + bolt (91611-B0845) change on Cara yesterday and today.
I have no idea when the fuel filter was last replaced, but the new one looks so pretty in comparison! $10 for 23300-38010 from my local Toyota dealer.
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I changed the T-stat hoping that would resolve my odd over-cooling issues I talked about a few months back and had other observations on:
And I do think I found the cause. When I took the thermostat housing apart I found the upper gasket had been bent at this crazy angle; no idea how that happened. From reading on here it sounds like if that gasket is misplaced or doesn't have a good seal, then coolant will just rush by and the system behaves like the thermostat is allowing coolant to flow. Which I think would explain my system behavior well.
Did a test drive this morning around the neighborhood after replacing the T-stat and gaskets and a quick system burp. The system showed system temps between 170F (bottom of a 300ft hill) to 190F (top of 300ft hill) on my third-party gauge in overcast 60F PNW weather; and my OEM temp gauge was around 1/4. Previously I wouldn't get above 150F or have my OEM temp gauge move at all. Since I installed the temp gauge three months ago, I have never seen 180F+. The Aisin THT007 is rated for 190F; with a possible temperature differential between the thermostat and then flowing through my upper rad hose (a couple degrees?), I feel like the system is working properly now. A proper highway drive would be a better test when I get around to it.
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Cara purrs on.