"It already has the new Radiator Cap, and Fan Clutch, and running the A/C with it parked it doesn't run hot. I have to be driving it for it to ever get hot."
OK, Now we are getting somewhere! If it can idle with the AC on in 90+ ambient for 30 minutes without getting hot, the water pump, thermostat, fan clutch, and electric fan are not the problem!
"My next step is to replace the thermostat again with one from the Dealership this time..... and if that doesn't work I will probably just try to sell it."
Normally I would say don't waste the time and money on the dealer thermostat at this point, but if the the thermostat you have does not allow enough volume flow of water it could be the problem. Some people here hadnon OEM thermostats that allowed to little flow, so it overheated, or allowed too much flow, so it would get hot enough in the winter, or cycled the thermostat cause wide temperature swings so that might be worth a try.
Other ideas are:
Torque converter slipping, not locking up at highway speeds, would add heat load to the radiator.
Not a plugged exhaust system, but a partly restricted one that becomes noticable, and has excess cat converter heat backing up back into the engine only at higher rpms and loads, like highway driving. Just have someone give it the gas to say 3000 rpm while you check the tail pipe for good flow at high engine rpms to see if this might be part of the problem.
This one may be debatable, but the computer uses the data from the O2 sensor to control engine timing and the fuel to air ratio. If it is making the engine run too lean at highway speeds it would cause overheating. There has been some discussion here about fooling the computer by biasing the O2 sensor. Since yours is a 96, with OBD-II, and has two O2 sensors (correct?) then a biased O2 sensor would set off a trouble code and check engine light, right? Have you check for trouble codes, on the chance the engine trouble light is burned out? Autozone does OBD-II scans on 96 and newer vehicles for free!
Here is an idea, if the TCU side of the TPS is bad, it could be making the engine shift late and be causing the engine to run in a lower gear than normal making the engine and transmission run at higher rpms and thus run hotter. Check and post the engine shifting rpms, and coasting rpm at various speeds, especially the highway rpm /speeds where it overheats.
Do you know how to tell if the TC is locking up? It is suptle, can seem like or feel like a fourth or fifth gear shift. It usually locks up at a steady coasting speed of 40 to 45 mph either in third or fourth gear, but that all depends on a properly working TPS, TCU and good signal wires.
Mine was shifting way too early and so I had no power (doing the opposite of what I just described as a possibility for yours. TPS can bad in many different ways causing a variety of weird problems). Also mine would not downshift at all. Turned out to be a $35 TPS fix.
It is possible to wire up an LED to the TCU to tell you when the TC has locked up!
I just reread your first posts, so a few more final thoughts. Did you replace the bypass hose, are you sure there is not a tiny leak at the heater core hoses on the firewall, etc....
On the AC, the AC compressor should be cycling some if it is on Max AC with the cool lever slid all the way to the left, and should not cycle in this weather (heat) if it is on normal or third AC option (I forget its label name). If it is not cycling while on Max AC, then it might be overcharged with too much refrigerant or the damper door that selects inside air recirculation (with no outside make up air allowed in) then it may not be working. The damper door is vacuum actuated.
An over filled AC system (as I recall) can cause overheating, but I would expect such an overfilled AC system to cause more rapid overheating, idling in the driveway for 30 minutes, not on the highway, unless it takes the added load of highway speed (tranny and engine working harder, producing more heat) combined with an overcharged AC to finally reach the tipping point for the cooling system.
I would check the AC system high and low pressures to see if it is overcharged.
All that said, I am wondering if you bought a minimal capacity radiator that is OK until you hit it with everything load wise.
The guys at Radiatorbarn.com sell a high quality, 100% brass, brand new three row radiator for some rediculously low price like $120 delivered. I have one I bought 2 years ago, never had a problem since.
