My 2000 Tacoma with 137,000 miles, has started running hot today....Like suddenly. It does it dramatically when the AC is running but not so much with it off. Bottom radiator hose is hot, so the thermostat is not sticking shut. There appears to not be enough pressure in the top hose when its running, but I am not sure.

I suspect the water pump, but can it work just partially? and go out in degrees?(no pun intended) And why does the AC running hasten the temp rise so much?

Normally, when the water pump goes out, it dumps water on the ground. It should have been replaced when the timing belt was replaced anyway. (Assuming it's not a 22r) The thermostat could be stuck partway open, allowing a little coolant through, but not opening enough to allow full coolant.

The AC system works by taking the heat from inside the cab, and dumping it in the condenser in front of the radiator. This will make the engine run hotter.

I ended up taking it in for diagnostic check. They said the fan clutch was bad. They replaced the fan clutch but it still runs hot when AC is running or when running in lower gear, as in up steep hills. Otherwise it stays at the mid point on the temp gauge. I also replaced the thermostat myself.

When I took it in for a timing belt change last year, the mechanic said I had a "Chain" and it was good like forever...almost.

I thought I would just take out the radiator and clean it good, but I'm stuck. I can't get the 4 nuts off the fan/fan clutch assembly attaching it to the water pump pulley. I know I have to hold that pulley still somehow. There must be a common solution to this problem. Any mechanical wizards got any suggestions?