I built a stroker motor last summer and my '90 XJ has been running flawlessly. I have a 3 core radiator, high-flow thermostat (with a bleeder hole drilled), high flow water pump, and hood vents, and it has always run at a constant 195 on both temp gauges until recently. I also switched the electric fan to a toggle switch, but I never have to use it unless it's a hot day and I'm running the AC.

Recently, it has started to run hotter - about 210. I checked my coolant level and it's good. I did notice that if I turn the heater control to heat, the temp will RISE about 10 more degrees - which seems REALLY weird to me. Temp will also rise a little more under load.

I can understand a bad fan clutch, head gasket, etc., but I just can't think of any reason why it would run hotter when I put it on heat. Any ideas?

 

210 is not overheating. On my 1997 and 2001, 210 is right in the middle of the gauge and the Jeeps usually runs from a needle width below to a need width above (stopped in traffic). I'm not sure why it would run hotter with the heat on. The water flow through the thermostat housing is likely changed if you still have the heater control valve by the firewall. Sounds like its better to have the engine a little warmer when you want heat.

 

That heater control only opens a door under the dash.....

Eliminate your heater control valve under the hood anyway before it explodes and dumps all your coolant.