Odd Overheating

Well Ive noticed a new quirk on my 98. It happens if I run it for 20 mins or so, so it's been ~210 for a while, and park it to say run into a store for 15-20 mins. I will come back out, and start it up, and the guage jumps up to 220-240 , then drops right back down after getting air and coolant moving again. My first problem is the electric fan doesn't kick on at 210. But I have a switch on the fan to keep it from overheating, so it's fine when it's running.

What would cause a vehicle to get that much hotter as it sits? Plugged radiator?

thats exactly what i was thinking

Think about it for a minute. If your coolant temp is 210, and the air flowing through the radiator is 70-80, it stands to reason that the actual engine temperature will be a good bit higher than 210. So, once the coolant is not circulating and being cooled, it will absorb more heat from the engine, resulting in the briefly higher temps you see.

The iron block gives off heat as it sits much like a rock sitting in the sun all day gives off heat at night. The temp gauge indicates the temp of the COOLANT, not the block. When the coolant starts moving again the correct temp is indicated.

After you've been running for a while try turning off the engine and then just turning the key to ON, if your temp seems to rise, that's why. Don't ask me about the technicals on this cause I don't know. I just know that it happens.

The phenomenon of "heat soak" is what you are experincing, it's normal....

Once you shut down a hot (Normal op temp) engine, the coolant then has time to absorb more heat and transmit it to the coolant temp sender, thus giving you the high reading.

As soon as the engine is started again, the coolant is flowing and cooling the engine, and the temp on the gauge drops.....

I notice it all the time with my XJ. If I restart after about an hour, the temp is actually fairly cool on the guage....

As long as it isn't overheating while it's running, your fine.

Your fan may not kick on just after start up, once the coolant moves into the radiator, it's temperature drops drammaticly before it gets to the fan switch.......

If it's normal, why is it jumping up high enough, long enough to give a check guages warning, then a CEL? I need to pull and see what it is, but it's obviously a warning on high temps or something related.

I plan on a new water pump, tstat, and radiator this summer anyways, just curious cause Ive never had this problem before a few weeks ago...

The ECM reads the temperature sensor and starts the fan. Would suggest you replace the temperature sensor that screws into the thermostat housing and the fan relay inside the PCM on the passengers side fender. If either one of these is not working correctly; it will stop the fan from coming on at 217*.

The motor cools with convection, the coolant actually keeps flowing after the motor is shut down. Something interrupting (slowing) the flow, could be your problem.
The outside temperature (ambient) also has a lot to do with how well the convection cooling works. There isn't much air flow after the motor is shut off.
One reason many vehicles have the fan on an always hot circuit rather than the run (key) circuit like the XJ does. The fan cooling the coolant after shutdown actually increases the (convection/advection) coolant flow. The hot coolant goes up and the cooler coolant goes down, causing circulation.

Fan relay is fine, the fan runs if I disconnect the AC I think it was? It's been checked, anyways. Ive also replaced the CTS for other reasons, so I don't think that's it. Im gong to recheck the wiring, however.