So still had some hot running issues today. Drove the 300D into work, and on both the way in and the way out, temp climbed fairly quickly to just under 100C but then halted there. No movement in either direction really more than a degree or so. Seems baffling. When I tried burping the system as I had done previously, the temps didnt really drop at all, unless I turn the heater on in which case they drop pretty quickly.

So my initial thought was a lack of coolant flow through the block and radiator. But the only thing that could cause that would be a seized or otherwise broken water pump right? And I had the water pump replaced around 5k miles ago, so I’m not inclined to think thats the issue. The other thing is, if the water pump wasn’t pumping, why would temp hold so steady at 100C? The only reason I have to believe the water pump may have failed is that when I did the full coolant drain and flush, when I filled the coolant back up, the block probably wasn’t filled before the car was started so the pump would have been running dry, but that shouldn’t really matter at all right? The pump design is just a shaft with an impeller on the end of it. Is there anyway to test that the pump is circulating fluid other than say, taking the upper rad hose off and running the car?

The next bit of confusion is that I can drive the car 30+ miles with it around 100C the whole time, but when I park I can pop the coolant overflow tank cap off with no drama. That tank isn’t really part of the coolant circuit (Coolant tank has a big hose to lower rad, but then the hoses go T-stat housing outlet to lower rad, upper rad to T-stat inlet), so hot coolant wouldn’t be flowing through there on it’s own right? Because it’s not part of the circuit?

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So where I’m at now is a state of utter confusion. I have another water pump on order and if I dont have answers by the weekend (I have another car I can take into work for a day and then I’m having LASIK done thursday so no car needs till Monday), I’ll throw it in, but I really dont feel like doing all that work to replace a component that was recently replaced.

Oppo, any help? Any advice? I’d love to be missing something obvious.