Temp. gauge sender gone bad?

I've overhauled the cooling system in my 89 XJ, 2wd, 2dr, 4.0, 5sp, A/C. At one point during "burping" my buddy (the previous owner of 12 years) yelled: "Hey the needle pegged all the way over then came down slowly to center." upon starting it from stone cold.
So far now no matter what I've done, like jumping the electric fan to run all the time and removing the T stat, it shows running temp at 3/4 of the gauge after it warms up at best. Even at a cold start up having not been run for days the gauge still reads between 1/8 and 1/4 the moment it's started. (It was 60 degrees yesterday here in Fl. so the gauge should be bottomed out right?) In many years of fixing cars I've never seen a sending unit fail so it reads "higher", I've checked the ground straps for continuity and other than the high gauge reading it runs fine. Ideas? :banghead:

Were you burping through the gauge sender port in the rear of the head?

Did you put any sealer on the sending unit? Single wire senders ground through the body, so if you used a sealer that is non-conductive you will screw up the readings.

You can buy a number of resistors at Radio Shack to test the gauge: 1365 ohms for 100 degrees; 93.5 ohms for 220 degrees; 55.1 ohms for 260 degrees.

Note that the temp sending units are somewhat easy to break.

Good luck.

Thank you, did the FSM burp, run, cool, run, cool (squeezing the upper hose) over 2 weekends. The senders never been removed. Drilled a 1/8th hole in the T stat per someones recommendation and had that hole placed topmost when assembled. Will check gauge function with recommended resistors. b5

I've got a NAPA TS6655 to replace the old one and will pick up resistors for testing the gauge (I was getting diodes for the fan control circuit anyway) this weekend. I read elsewhere here that some sending units are off 20%? I replaced the thermal fan switch on the radiator with new NAPA and it has yet to activate I'm wondering if it is set at to high of a temp. (I've been testing it for continuity when the gauge reads over 230.) Thanks again Joe I really like this little XJ. b5

Replacing the sender worked, reading normal now.