Gauge says it's hot, but it's not

First thing, I did search. Came up with Nada.


Second,

Engine gauge says the engine is overheated on a random basis. Most of the time the gauge reads straight up. but sometimes it will get a little warm and hang out. Then it'll jump way up in temp and stay there for a while. Nothing I seem to do gets it to cool off the gauge reading. and the real strange part is that the engine isn't hot, not even close to overheating. It cools down eventually, and it's never boiled over.

I've replaced the temp sensor in the engine bay. I thought I read that there was two of them but I can only find the one up front. Is this a computer issue?

Any ideas?

98 Sport. 4.0 No codes are being thrown. It is slowly leaking coolant from a freeze plug. but I keep the overflow bottle filled.

Check your coolant level pronto. That's a common behavior if your coolant level is really low. (Don't ask how I know)

I am not sure if it is the same on a 98 as a 95 but my 95 has a second temp sensor near the back driverside of the head. That one is for the gauge and the front one is for the computer. My gauge reads 225 once it heats up. If I use my inferred heat gun it will tell me that nowhere on the head, block, or thermostat housing is it over 195 degrees. So my gauge is off by 30 degrees I guess.

Also, air bubbles have a way of getting trapped at the rear temp sensor and making the gauge spike untill the bubble gets pushed out. You might find fixing your leaking freeze plug is all you need to do.

Best to verify this with an infrared point and shoot thermometer pointed directly at the thermostat housing.

If temp is verified staying constant and you have replaced the temperature sending unit, you could have problem with either the connector on that sending unit, the wiring, or the coolant temperature gauge itself.

97 up only have the one located in the T-Stat housing. Is the coolant low? Does the "overflow" tank require refilling? Air trapped can cause this symptom. If it is losing coolant and the tank goes dry, then when the engine is shut down and it attempt to pull coolant from the tank to refill, it draws air. Also, make sure your T-Stat has the bleeder at the top of it. Most, if not all, aftermatket T-Stats do not have the bleeder. An eight inch hole drilled in works just fine. I usually drill four of them, but then I'm a belt and suspenders sort of Engineer...

Your leaky freeze plug is causing presure to drop and the coolant to boil. Fix that So that you can diagnose for other issues.

OK, I was thinking the plug may be affecting it. I keep the overflow full,

Does the computer come into play with the temp gauge anywhere?