Driving along and all is normal. Then it jumps up in temp to one notch below pegged and does it in less than 2 seconds. I shut off air, keep driving and in about 5 - 15 minutes it drops again from hot to normal in less than 2 seconds. Never get any steam or antifreeze smell. Any thoughts?

 

Sounds like your gauge my be giving you an incorrect reading...

In reality, I don't think your coolant is going to change that drastically so quickly.

Verify your temperature with an infrared temp gun.

 

I think you have a gauge or sensor issue. The sensor would be the easiest place to start.

 

I think your right. I'll try it tomorrow

 

Bad temp sensor. The PCM in those years drives the gauges and it can exaggerate the gauge reading by driving it nearly full-right when it thinks you are overheating....to get your attention.

 

I changed the thermostat and temp sensor and it still acts up. Today I figured out the a\c doesn't have anything to do with it. It ran along at the normal temp (1 line past 210) then jump up to the first red line in about 2 seconds again. I just kept driving and it eventually dropped back down all of a sudden. I checked the top hose temp when it said it was over heating and it was 219. I check the wires for continuity to the PCM and they were good. The book seems to show only 2 outputs from PCM to the whole instrument cluster so I don't know what to make of that. Also don't understand why there is a return wire to the sensor. I figure its either got to be a bad wire from PCM to gauge or the PCM itself which I can't afford, especially if I'm not sure it the problem. What happens if the PCM doesn't get temp info? Can I replace with different gauge. Any tests I can do to see if it is PCM?

 

Very well could have air trapped in the cooling system. Park with the front end up hill and pull the cap off the radiator (while the engine is cool) then wait for the T stat to open and check or fill your fluid while holding the throttle partly open. Good luck.

 

^^^^ this x2. I just did mine this morning an make a habit of burping the radiator once every six months or so for good measure. What happens is the air pockets let the water evaporate an turn to steam. When it goes past the temp sensor it will cause it to jump high an then when u start pushing water past again it drops to a normal reading. This should fix your problem.

 

That's a data bus. It allows the PCM to talk to the processor in the cluster.

 

sub'd. my jeep does the same thing. but only after a steep hill climb, on the highway.

 

I checked for air, didn't seem to be any and it's still doing it. My brother says he had to pull the hose off and fill his system on his older cummins truck to get the air out. I talked to a computer repair place and they said "maybe, bring it in and we'll check it" I guess it's my next step if I can't find something else. Does anyone know if those computer diagnostic plug in things read water temp. I would like to be able to read what the sending unit is saying verses what the computer says it's saying.

 

Unless it's a code-only cheapie it should read live data.

 

When you put the t stat in did you put the correct side up? Did you put it in correctly? Try turning on the heat when you are filling the radiator. You have air trapped no doubt! Both radiator hoses should be the same temp as should both heater core hoses with the heat running. Don't waste your money at a computer tune place. IT'S AIR IN THE COOLING SYSTEM, take our advice or stop asking for it.

 

My jeep overheated after driving 1 block... I had,a clog in the radiator from corrosion from the old owner who never ever put antifreeze in... -.- anyway it blew a small hole si thus, instant overheat

 

When my XJ is one line over 210 she's boiling water. After my radiator replacement from jeep she runs one line under 210