Only overheats above 50 MPH?

I'd appreciate any suggestions anyone can offer about this. My daughter, who's second baby is on the way, got a '93 3.0 Voyager yesterday. The transmission went out on her explorer, and they only wanted $250 down for the voyager. We checked it over, and everything seemed fine. She didn't even get it home before it overheated. When she pulled it over, the coolant started boiling and a little overflowed the resevoir but not much. Once it cooled, I checked the water... it was over a gallon low. Since it hadn't overflowed this much, I hoped that was the problem. However, over the course of the rest of the trip home, we discovered that it only overheated over 50 MPH. The upper and lower rad. hoses were both hot, so no clog. I squeezed the upper and felt the rush when I let go. There's no steam from the exhaust, or water in the oil. My only thought now is maybe a partially stuck t-stat. Does anyone have any other ideas?

Thanks

I tend to think that your problem is not with the thermostat, but rather with the cooling fans not coming on when the radiator is hot.

To debug: turn your AC off (because the cooling fan may operate on low for AC requests) and let the car idle for about 10 minutes. Watch the temperature gauge. It should take about 3 minutes to rise to the normal operating temperature (water heating in the engine block) and pause at that temperature. At that point the thermostat opens and the hose to the radiator will begin to get warm. If so, the thermostat is good. If fails to open, the temperature will simply keep rising and the upper hose remains cold. It will take another 5 minutes to heat all the radiator water up to the point the cooling fans should come on. If the tempature begins to rise after that (about 5 minutes), expect the cooling fans to come on. Don't overheat the engine.

If the cooling fans didn't come on, the usual suspects are either the temperature sensor switch (located on the bottom of the radiator) is bad or cooling fan relay (located in engine compartment on the left side in a box) is bad. There may also be a fuse (in that relay box) for the fan motors - can't remember for sure.

To debug the temperature sensor switch, you can detach the connector, and place a paper clip in both sockets to jump the switch. With the ignition on, the fans should come on. It will mean the relay and wiring to the motor is good from that point on. That leaves the switch (or there is no voltage from the ignition to the switch - ??fuse??) as the component that's bad.

If still no fans and you can locate the proper relay: try swapping that relay with another relay in the box with the same part number.

Hope that helps.

Thanks for the excellent suggestions! Before I saw your post, I decided to pull the thermostat (since it was easy and I wasn't really sure what to try first ;) ). When I pulled it out, The whole bottom of it was falling off. I happened to have a spare for my Explorer which turned out to be the same part #, and it seems to have fixed the problem. We drove it for about an hour today at sppeds up to 68 or so and were unable to make it overheat. The fan appears to be comming on at about the right time, though I'll keep a close watch on it... It already looks like someones been messing with the wiring.

Thanks again!