Thermostat's are cheap, and malfunction often. I replace mine with new Anti-Freeze every 2 years.
If a thermostat goes bad, it usually doesn't show until the change of the seasons, when the cooling load for the motor changes. Your Thermostat might NOT be closing far enough, probably unnoticeable during the summer, but now that its fall it does make a difference.
Most XJ's measure temp with a separate sensor in the head, where the temp changes the most, even when cooling properly. Most vehicles measure temperature at the thermostat, where the temp doesn't change at all, provided the cooling system is working properly. (I'm referring to after warm-up). Thats why you see the temp gauge go up and down some on an XJ, with engine load, while you never see it move on other vehicles, unless your cooling system is malfunctioning.
Runner the motor cooler than designed NOT only hurts gas mileage, it ALSO affects wear and deposits on the motor. Carbon collects on cooler parts (valves) and does NOT get burned off, the motor is designed with clearances that change the temp of the engine. There are lots of dyno test that show many motors make more power running a little hotter than designed temp.
I know lots of people claim that running the motor cooler, "TRICKS" the EFI into making more power, but they're explanation of how this works never holds up and I have yet to see any of them to provide the slightest evidence that it does work.
Sure, there may be some pros that run their Super Hopped Up motor cooler than stock, probably because running it under heavy load, racing or hauling, pushes it to overheating, and runner cooler thermostats gives them a little more margin and cools the motor off during low load and idle. That not the case for most of us.
