Chrysler Repair: Temp gauge/temp control: 2.5L 4 cyl engine, 90, lebaron convertible, radiator cap

Question
1989 Lebaron Convertible with 2.5.
the temp gauge goes up like a thermostat sticking hold on Hot for a  time. (never same amount time) them drops like the thermostat opened . This goes on some time a lot other not so much.
I have replaced the thermostat 2 times. I have replaced radiator. I have replaced sending unit the gauge 2 x as they was used I wanted to makes sure. I also replaced the electronic block on back of the dash. when the temp says it's hot, the engine does not act hot. you can remove the radiator cap . the fan is working right goes off and on like it as from day one. Out side temo seem to play no role as it does this in the 20's and the same in the 80's
I am at a lost.


Answer
Hi Larry,
Are you familiar with the special procedure that you have to use when you refill the cooling system on the 2.2/2.5L engines? If not, there is a need to refill it in such a manner that you don't trap air in the thermostat housing box. Notice there is plug on the top of the box. The procedure you should follow now is to begind by starting the engine and putting the heat/ac control lever in the hot position then shut off the engine. Without removing the radiator cap drain out about 2 quarts of coolant from the radiator and collect it for reinsertion. Watch to be sure that the overflow bottle empties out first which shows that the system is air tight. Then close the radiator drain. Then remove the plug on the top of the thermostat housing. Then start adding coolant to the radiator until the level of the coolant raises to fill the thermostat housing to the top of the hole from which the plug was removed, thus assuring that there is no air trapped in it. Then put the plug back in the hole. Then refill the radiator all the way and finally put any leftover coolant in the overflow bottle. Put on the radiator cap and you are ready to roll. It may well be that the thermostat is not functioning as it should because of the air bubble trapped in the box.
Roland