The TJ is a quirky little beast with regard to the thermostat. If you use a fill funnel, you can watch what happens when the thermostat opens on most of them. It it will be fine, thermostat opens, then it will puke out a couple of quarts of coolant from a big air bubble. If you drill a 1/8" hole in the flange and put that on top, all that nonsense goes away. You can watch the thermostat open by monitoring the coolant in the fill funnel which is static before it opens, then it will drop some when it opens and you can see movement in the coolant that tells you it is circulating. Simple, easy, very best thing since sliced bread.