Fixed high temp gauge reading

Try cleaning the electrical connections to the sender.

So my 91 had high temp readings during the last two summers. NBD didn't overheat so I drove it. Well the other day 85 degrees out and traffic, it got real hot. Finished the drive and turned for home. Stopped for a quick trip to Costco (under $100 so it is quick) and got back to a quart or so of coolant under the Jeep. Ok, just get me home (20 miles), nope temp crept up and over and over so I pulled into a station and let it cool and filled the overflow which was sucked dry as the engine cooled (not really I kept filling it).

Put the new radiator in the day it arrived and noticed that the temp gauge was reading about 150 cold and about 3/4ths warmed up. The gauge was as all the rest before this reading straight up as expected, except on hot days. I did notice that the thermostat opened at about 3/4ths now. Infrared readings of the engine confirmed correct operating temps. What the heck?

I started by trying to get the temp sender out It is kinda hard to get to and the Jeep was not in the driveway, so I just cleaned the threads on the connection and reinstalled the wiring. Jeep reads as it should with the new radiator.

I cut the old radiator core open and it was clean. I did not open the tank ends where I suspect there was corrosion plugging the tubes. Whatever the Jeep is fine now, even after the overheating situation. If this had been some engine with an aluminum head I would have killed the engine. Sometimes iron is good.

Hope you don't mind if I jump in. My temp gauge has been reading sporadicly also. It will read high for a minute or two then go down and be good for weeks. Now I notice it reads low for awhile, then goes back to normal. I bought a new thermostat when I noticed the problem last year but still haven't put it in (I know, I know). I've been thinking that it might be the sending unit instead of the stat. Where is it the sending unit located? I might try a good cleaning like you did, before I replace anything. Thanks, John.

All the way at the back of the head. Look at the back left (right from the front of the engine compartment) corner of the valve cover. It is just outside that. Mine has a 90 degree bend rubber cover over the connector, a single wire hanging from the harness on the firewall.

mines been reading about 20 degreese too hot. could this be the problem?

my 90 has always read at the 210 mark. on at the time i flushed my heater core. i noticed i had air in the system. so i found this. http://www.olypen.com/craigh/cooling.htm
and it now never gets up there. about 1/4 of an inch away

Mine has an aftermarket gauge (my Cherokee is an SE with and idiot light)that I had put in when I bought it 10 years ago. It's always read low...right at 190. I've replaced the sender about 3 times in 10 years. Shuold've though of cleaning it. It is difficult to get to.
I also did infrared readings and the temps look pretty much where they should be, about 200-213 at the thermostat, maybe 10 degrees hotter back where the actual sender for the gauge is. The upper radiator hose reads about 165 halfway bewteen the radiator and the stat housing.

i hate the ones with the lights. who wouuld ever come up with that. i meen if it had a nice lite like the gas gage that if it gets high a little red light comes on.