To avoid the heat of the core in summer fighting with the evaporator 2 inches away.

 

Got the old water pump off, but haven't gotten the new one on. There was definitely water coming from the weep hole, so it appears that it is bad. The bearings themselves, though, seem fine. There's no visible corrosion on the impeller, no noises when I spin it by hand, and no movement. So it makes me wonder what the failure mode of a bad water pump is? I was expecting to find rusty bearings, or be able to feel that they had gone bad.

 

With coolant weeping out, you haven’t wasted your money on your new water pump. I hope this solves your cooling issue, but I would think that if it was your main problem the original pump would have looked much worse.

 

The weep hole leaking tells you the bearings have worn enough that the seal has started leaking. Once the seal starts leaking you get water in the bearings and they don't last very long. You caught it at the right time but that is not your cooling or overheating problem.

 

Replacing the radiator water pump didn't change the overheating behavior, so you were both right. I'm not sure what to look at next. I attempted to do a pressure test overnight, but the gauge was broken on the kit I borrowed from the parts store, so I couldn't get any actual numbers. There was a satisfying fwoosh and rush of bubbles when I released pressure this morning, but I have no idea if it was down ten pounds or rock solid.

Everything in the cooling system is new at this point, except for the heater core. Extinct's thread linked above indicates that a clogged radiator is the most likely cause. It's a brand new Nissen's, but that doesn't mean it isn't bad from the factory. How do I evaluate it? What should the hoses feel like at temp? I'm going to get a different pressure tester and attempt that test again, as well.

 

Will it idle without overheating? Overheats on the highway? You have the thermostat out?

 

I just let it idle without touching the gas and measured everything with an infrared thermometer. It took about ten minutes to get up to 220 degrees with no thermostat. I noticed that the hoses didn't get warm until it was above 200 degrees. When it got to 220, I hit the gas and brought it up to ~2500 RPM to see if the fan would cool it down. It didn't. It quickly rose up to 230, at which time I turned it off.
But here's the kicker: the thermometer never read temps above 190. I tested it again a few hours later after everything cooled down. Idling, the temp sensor reported 220, but the hot side of the radiator only showed 190, and the cold side showed 96. I took it for a very short drive and the temp gauge shot up to just under redline. My OBDII reader reported 253. When I pulled into the driveway and shot the metal thermostat housing, it was below 200. Can I have a bad temp sensor? When this all started, I did have actual overheats where the car vented large amounts of coolant. I think my original radiator was bad, and my water pump was on its way to failure. But having replaced those, am I now just being lied to by the temp sensor?

 

You could have a bad temp sensor, sounds like you might based on your IR readings. Remember the IR is going to read 10ish degrees lower than internatl coolant temp, but it sounds like your are reading 30-40 degrees different which is too much. You can run an external temp gauge and many of us do. At this point I would guess it is between the temp gauge or the fan clutch.

 

I live in a city, so I don't have a good way to get to a highway quickly. Now that I know it isn't actually even close to 250, though, I might risk a trip to the interstate to see what the temperatures look like. I also have a new temp sensor on the way.