Temp Gauge behavior when over heating

My 98 XJ has been getting hot lately and I've done the research on all the prescribed fixes. My question is about the way the temp gauge reads.

When running on cooler days, it runs at 210, but when running on the highway on a hot day with AC on it runs about 220 deg. Then after a while which sems like out of nowhere, it shoots up to 250 and the check gauges light comes on. So I turn off the AC and turn on the heat and it takes about 30 secs, then the temp drops fast back to 220.

I'm wondering that is normal behavior? With other vehicles including a 91 XJ I had, the temp would creep up slowly and then when I would turn the heat on, it would go back down slowly.

Would a bad temp sensor make that happen?

Hopefully this makes sense... Thanks in advance

That's pretty normal. The gauge jumping up like that.

On a hot day with the AC on mine usually runs about 220 or so. When I shut the ac off, it runs back down to about 200 to 210. .

Time to overhaul your cooling system.

i wouldn't call that normal. maybe normal for a faulty cooling system, but i wouldn't wanna risk it not coming back down when your away from home. time to start working on the cooling system.

check the steel heater control valve lines, mine we're completely clogged, causing that to happen often...

98's don't have that valve.

Also I believe they use ONE sensor for everything. Fuel mixture, electric fan, AND gauge. Perhaps there's something wrong with the gauge or the sensor? Jumping around isn't normal. Double check the temp against a thermal, contact, or laser thermometer!

Had the same problem with my 97 (both of them).
Turns out the problem was an old radiator. The corrosion had built up over the years causing a slight obstruction in the overall flow. This reduced flow would occassionally cause the radiator hose to collapse since the water pump was pulling coolant faster than the radiator could supply it. This momentary 'starvation' would cause a spike in coolant temps since it's no longer freely flowing through the system.
Once the hose opens up, the overheated coolant exits the motor and hits the radiator sending the gauge through the roof. This heat spike only lasts a short time since the coolant is once again flowing... that is until the next time the hose collapses.

Get yourself a new radiatior to restore a normal coolant flow and everything will get back to normal.:cheers:

It took me a couple of months to track down this problem since I (and everyone else) thought it was something else. [waterpump, t-stat x2, heater hoses, radiator hoses, cap, flush]