If it takes a long time for the engine to reach operating temp I would suggest the thermostat may be stuck open or partially open. All 4 of my jeeps, all 97's and 98 take no more than 8 minutes of driving to reach operating temp. If it's taking like 20 you have a problem and it's probably the thermostat. Thats two bolts, new thermostat, new gasket and about 20 minutes to fix. Make sure you get the thermostat with the bleed hole and put that hole at 12 O clock when you install it. I've had alot less problems just getting the tstat and gasket from the dealer, they fit and work right, I've had aftermarket gaskets leak or not seal correctly, even the felpro ones.