I've been down this road.... Sometimes it's the radiator, sometimes it's the water pump, sometimes it's the thermostat, sometimes it's a lack of fluid, or a combination of all of these.
Your objective is to keep that temp gauge out of the red.
My advice to you.... Turn the heat up on high, turn down the windows.
This worked for me until I replaced all three items, it never overheats now.
Could be a couple of things:
- Water pump (check if it's good, if not, replace)
- Thermostat (three dollars to replace)
- Radiator (Flush, repair, and replace)
- Fan clutch; this is the fan connected to the engine, it could be bad (timing off) etc. and lastly check if hoses are good and do not collapse.
If these are not the problem, then it could be a more complex problem, keep in mind it is an old vehicle and they way it was treated could significantly change the reason it is overheating. Many other things that are nor reachable by hand can be clogged up or sensors nor working properly.
The closed style cooling system on early XJ models (1991 and earlier) can lead the hoses and fittings to harden and break because of aging. This results in small leaks and causes the engine to overheat.
Insufficient flushing of the cooling system or poor maintenance could result in a coked up system.
On newer type XJ's (1993 and later) the cylinder heads (mostly between cylinder 4, 5 or 6) and/or gaskets can break; broken cylinder heads can only be spotted by looking into the oil filling hole when the engine is running.