XJfire75 said:
I found some helpful threads on "burping" and drilling holes in the thermostat. but what do you guys think would be the best place to start? I'm thinking the tank and cap first then maybe the hoses coming off the tank. Need some opinions please.
The best place to start is the cheapest, so I'd suggest reasing those threads on burping and make sure you get all the air out of the system. This can be difficult to do sometimes, especially with the old closed system, so read those old threads and meticulously do all the things they suggest. For example, I may be remembering wrong, but didn't I read that you have to park those old closed system types on an incline to get the last little bit air out?
Another easy/cheap thing is to visually inspect the front of the radiator to see if you have a bunch of dead bugs/grass seeds/dust/misc junk clogging up the openings. Flush that stuff out from behind with water or compressed air.
As far as the overflow bottle and hoses, you should probably replace the pressurized cap on the bottle but otherwise, unless you see visible leaks, they would not be causing overheating even if they're bad. Replace them if they're old and worn to save future grief, but don't expect that to help with the overheating problem.
After that, I'd check the fans to be sure they come on when they're supposed to. I presume the closed system still had 2 fans like the newer models, with the electric fan coming on about 210 degrees.
I, too, was a little confused when you mentioned the tank cap, but I'm guessing you mean the overflow bottle cap. If there really is a cap on the radiator, you might want to compare your cooling system lay-out to either the manual or to another pre-92 XJ. Since you don't know the history, perhaps a prior owner started to convert to an open system but didn't finish, leaving you with a hybrid. I really don't know what would happen if you replaced a closed system radiator with an open system one, and left everything else the same - but I wouldn't be surprised if it didn't work right.
T-stat and water pump and radiator should be kind of a last resort, but let me clarify something you mentioned - about "drilling holes in the thermostat". The stock T-stat has a weep hole in it, but many auto parts stores will happily sell you one the same size without the weep hole and say it's the one for your truck. This happened to me when I was going through my over-heating problems, and I did have a modest improvement when I put the right one in - not much, but enough when combined with other things to fix the problem.
Like Kevin above, I'd recommend the stock 195 one from the stealership. I suppose it would work to drill a hole in an aftermarket one, if you knew exactly what size hole and where to drill and that everything else was the same, but it's probably safer just to buy the stock one.
On the water pump, if it's going bad you should notice a loss of fluid from it after warming up the engine. Even if it's not bad, you might get some improvement from a higher speed pump (or the one on it could be the wrong one), but this is getting into more expensive fixes now, and I'd guess the odds are 10-to-1 that the problem is one of the cheaper things mentioned above.
Good luck and keep us posted what you find - as you may noticed from your searches, overheating is a problem a lot of us have faced and it can be very frustrating to figure out how to fix it.