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That is what so frustrating. I can operate the car at 80 mph (interstate) on an 80F day and it actually runs cooler than it does at 30 mph.
The higher highway speed increases the ram air effect and the cooling on the radiator. This is compensating for a partially plugged radiator. At slower speed of 30 mph there is less ram air effect, the electric cooling fan turns on but it still cannot pull sufficient air across the radiator to cool the engine properly. Because the radiator is partially restricted in effect you may have a radiator with heat exchange surface area that is 1/2 of what it needs to be. You need to replace the radiator.

Going to an external transmission cooler is not really going to solve the overheating problem? Where are you going to mount it? In front of the AC condenser? If you mount it there the heat of the transmission is going to cause problems with condenser heat exchange and not allow the AC to cool the interior of the car properly. If you mount it in another location then you will have to provide a fan to draw air across the transmission heat exchanger when you are driving at slow speeds.

One way to test a radiator is to remove it and take it to a radiator shop and have it flow tested. Of course someone at the radiator shop will have to know how to do this. I doubt if you can find someone knowledgeable today to do that and find the proper specifications so as to check the radiator flow. And there is some cost associated with this.

You could remove the electric cooling fan assembly and shroud from the side of the radiator that faces the engine. Reconnect all cooling system hoses and make sure you have the proper coolant level and no air trapped in the system. Start the engine and let it idle until the thermostat opens. You could carefully feel the back side of the radiator and try and find areas which are cooler than others. You have to be quick in doing this as you do not want the engine to overheat. If you find many of these areas that is an indication of a blockage and no coolant flow. An alternate way is to use an infrared heat sensing gun and aim it and various areas on the radiator and find colder areas which would indicate blockage.
 

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I have had a reman water pump impeller pressed on backwards that would reverse the coolant flow. The thermostat won't open for a long time like this. The upper hose will stay cool for a long time as well.
I have also found a piece of cardboard stuffed between the condenser and radiator to block air that someone placed there at one time for better winter heat. I have also found a thick insulating blanket of 'tree fuzz' between the radiator and condenser. Take a good look between these.
Keep up with the diagnosis. We are coming up with ideas and possibilities, but we have not condemned any one component yet.
 

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Discussion Starter #23
First of all, thank you for the help. I am studying each and every reply.

My infrared thermocouple test went like this. Car started, fan went on (remember I had to do something as the fan would never go on unless the AC is switched on. Troubleshooting, sensors, pigtails, relays, nothing solved the problem so I guessed it is a computer problem, and they are impossible to get in México. I am hoping to install a thermostat housing switch from some OEM replacement and run that through a Tyco relay. There never has been a fault code unless I disconnect the water jacket sensor. Once the overheating thing is resolved I'll trust putting the new fan control in, otherwise it would be sort of like painting a house when it's on fire :-(

The test: I drove the car for five miles on level ground in 90 degree weather, and then shut it off. The gauge read something like 15% up from cold. I wrapped a rag around my forearm, took the laser temp reader, and stuck it through the fan blades and scanned the radiator, as much as I could cover, but in what I would guess to be a 2-minute scan at 6" distance I found no cold areas, The radiator started off at 205 degrees and cooled. I had to work fast and recover areas to see how much the cooling affected newr areas. When I ended, after around 5-minutes The radiator had cooled to 182 degrees. This is some pretty frantic work with the temperatures and trying to get that gun pointed at the corners past the shroud! Thank god, the meter "remembers" and locks the last reading.

Maybe I should remove the thermostat (original), put it into a pan of water then heat it to boiling and watch what happens. What would happen if it did not open up all the way? The car might work OK until a restriction caused coolant temp to rise faster than the water flow could contain it? I've never heard of a thermostat doing that. Jammed shut or open, but partial?

This would be a lot easier if I weren't deep in México scraping-by on Social Security funds. I am disabled, so what used to take me 15-minutes now takes two hours. I need that car as the nearest bank, the nearest ATM, the nearest gas station is 73 miles away over a badly rutted road. If I spent 348 dollars on a new radiator I would still have to drive 146 miles to go pick it up. I wish this was easier. But I am hardheaded enough to stick with it. I don't want to ruin a perfectly good engine. A million thanks for your help!
 

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KOG
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Never mind putting the thermostat into a pan of boiling water. Just replace the thing. I routinely replace them with hoses at least every 100K as they are known to have limited live spans. Themostats are cheaper than engines or even head gaskets.
 
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