It sounds like after all this, that your gauge or sender is at fault. Since your outlet temp is at 210 with the AC on, that is acceptable for XJs. And yes, the gauge is replaceable without replacing the whole cluster.

 

I know it was also the radiator and trans cooling lines, since they were leaking and the rear end was shimmying.

After servicing the transmission and replacing the cooling lines, the shimmy stopped.

After replacing the radiator, the rapid rise of the temp gauge stopped.

I am going to try to replace the sending unit, since it is easier to do. I am not sure if the grounding lug touching the sending unit would have messed it up, but it is an easier fix. When I moved the ground lug around so that it wasn't touching, the feedback in my radio lessened significantly, and the feedback through the radio when the door buzzer activated has disappeared.

 

Fords DO have trouble burping the air out after doing work on them. I have had the worst trouble burping a 5.0's after a rebuild. I can take a couple of days messing with them sometime.
I would go buy a mechanical water temp. gauge and hook it up either temp. or perm. and see what temperature the engine is really running. They are pretty cheap and easy to hook up
clint

 

I wanted to update this thread (I hate when folks just leave you hanging).

I did replace the sending unit, and it made NO difference.

Last weekend I went up to Fort Campbell, on the TN/KY border for a military team reunion. Heading up to Monteagle from the south is pretty intense for about 4 miles I believe it is. I was on the red line when I got to the top, and it cooled back down on the north side, within a couple of miles heading down hill. I was running about 3,000 RPMs heading up hill. The first half, the AC was on Max.

On the way back to Georgia, the southbound approach to Monteagle is less severe and I watched the temp gauge much closer. I approached with my AC off, running with the windows down, and the ambient temperature was 97* in the shade. The gauge reached 245* by the time I made the 3 mile up to the top, and I just kept going. Again, the temp was rising when I was at 3,000 RPMs.

Letting off the gas kept me going about 60-65 on the downhill side for about 3 miles. During this 3 mile run, the temp gauge ran all the way down to 140*. Unless I totally missed my science classes, cast iron does not cool off that fast.

So, 100* degree drop in under 4 minutes, heading downhill, with 97* ambient temperature. Does that indicate a bad temp gauge? Keep in mind, this is with a new sending unit, replaced before I headed out. I have a temp gun that shows 197* at the water outlet, and about 212-217* at the rear of the head and the heater control valve.

 

Water boils at 212 degrees F. If you have 50% ethylene glychol antifreeze mixed with water in your cooling system, your boiling point is higher. I don't know exactly what it is, probably about 230 or 235 degrees boiling point.

Jack