Could be a clogged radiator too. You can try flushing the system, having radiator hot tanked, or replacing it.

Water pump can be failing or corroded to where the fins are not pumping the fluid enough.

Running without a thermostat is actually worse. Will let system overheat faster. The thermostat acts as a slight restriction in the system to slow the flow slightly. This allows time for the heat to be dissapated from the fluid in the radiator. If it is free flowing, it runs through the radiator to fast to let the fluid lose the heat.

Thermostat temp range will not affect overheating. It only affects the warm up and constant engine temp. The engine is designed to run a 195 thermostat. That is the temp the engine runs best. if temps drop below the 195 range, the thermostat with start to close to bring it back up. When it is warm out in the summer, once it opens at 195, it should stay open. I run either a stock thermostat or the stant failsafe thermostat. The nice thing with the failsafe, is it will never fail in the closed postion, so no over heat from a bad thermostat. it is designed to open fully in fail mode.

So, start by flushing system. Flush radiator, heater core and then whole system. fill with a good mix of coolant. most like 50/50 mix. I run a 60/40 mix year round. If the flush does not help. then start looking for bad parts, one by one.

 

I hadn't considered the head gasket since there is no smoking,and the coolant level stays up.

 

The water pump is brand new,and I also run a failsafe thermostat. This XJ sat for about 4 yrs. with a bad motor before someone tore it apart to start rebuild. Sat for 6-8 months in pieces before I got it. I'm finding lots of little problems. I will try flushing the system. Thanks for the info.

 

Complete system flush,all new coolant. Temp still hangs around 240. This is the last major problem before I can get it on the road. Anyone have anymore suggestions/advice?

 

Yeah thats a typical XJ thing! I've had it run smooth as butter at 80mph with no issues other than when I would get to a point where I'm idle n then the temp would jump up reaaalllll fast...

 

Have verified temp with mech gauge or IR thermometer? Maybe a bad sending unit?

 

I have not tried a mech gauge, just going by in-dash gauge. Even though they are new, I will try replacing the temp sending unit and switch. From what I've read here,it seems to be typical for XJ's to run a little warm at idle. Is this true?

 

Some do. I would find someone with IR thermometer and check, before throwing parts at it. Might be running hot. Might be bad sender. Might be bad gauge or wiring. Try to verify before moving ahead. Plus, double check every component to verify they working right.

 

My xj is a marriage of parts from a '94,going into my '96. From the 94- motor and its acc., and since the '94 inst. panel had gauges,I swapped that in also. The '96 had idiot lights, so could it be an issue of wiring/computer not accepting gauge input(electrical problems are my kryptonite)? The temp sending unit and switch are for a gauge equipped system.

 

Well finally able to check with IR thermometor, and based on its readings,the gauge is giving false readings. Gonna try a new temp sending unit(cheapest part),and if it still reads false I will install a mech. gauge.

 

I installed a mechanical temp gauge, and now get correct readings.

 

See. Nicer then spending money on parts!