I have replaced just about every single component of the cooling system that I know to replace and I'm still having an issue with the gauge reading too hot. Initially my problem was that the coolant was boiling out so, I went to town on the whole system. Here is what I have replaced:

Water pump
Radiator
Thermostat
Coolant reservoir
Fan clutch
Hoses

After all of these replacements my gauge still climbs way past 210. Really it will climb as high as I'll let it before I turn it off. Last night we ran it for about 25 min (temp gauge hit 255) pulled the cap on the reservoir and the water temp was 200 even.

Am I wrong in thinking this is the coolant temp sending unit? Or is this something else entirely?

Probably important to note that we just rebuilt the head and it has a new head gasket. So it shouldn't have anything to do with that. Hopefully.

 

Time for some silly questions that are not intended to be insulting...

Did you accidentally leave a rag in a hose?
Did you purge/burp the air out of the system?
Did you use a proper ratio of coolant-h2o? 50/50 to 70/30
Is your engine block clogged with stop-leak from the previous experience that lead to changing the head gasket?
Is the thermostat opening, or is it stuck shut? Some are defective right out of the box.
Is the thermostat in backwards? (I don't know if it is possible in the XJ, but some cars and some t-stats can be reversed and result in no flow)

 

Are you using a Napa 703-1396 cap on the bottle?

 

I'll answer in order! And no offense taken, any help I can get I'll take!

1. No
2. Yes
3. Yes
4. Not that I know of
5. I've put two thermostats in this thing. Not saying it's the problem, just saying there have been two brand new stats in this thing in the last week.
6. I did actually check to make sure it was in right and yes it is.

On another note it may be important to mention that the system feels like it has an enormous amount of pressure, which may be normal. Upper radiator hose is hot and lower is cool to the touch.

@cruiser on the pressure cap, the cap came with the reservoir. Ordered it from rockauto.

 

Although not advised to leave it out for long, you might try removing the t-stat and see if this stops the overheating. While you have it out, put the t-stat in a pot of water on the stove, along with a thermometer, and heat the water up then note what temp the t-stat opens at.

 

I didn't have as long as I had hoped to have to do this but I ran the jeep up and down the road came back let it idle about 20 minutes and never even hit 210 without tstat. I'm planning on doing the boil test shortly

 

Did the tstat test. Opened up at 205

 

Stock caps suck.

 

did you ever figure out what to do to solve the issue? i see you said the tstat opened at 205.

 

you can check your gauge by unplugging the lower coolant sensor (CTS) and reading the ohms across the terminals. use the formula -0.1962*X+248.3 where X = ohms in order to get the temperature.

example if you got 344 ohms (-0.1962*344)+248.3 = 180.8 Degrees

 

I changed everything on my 89, and it ran hot.. Then we rolled it off a cliff.. It was upside down for an hour or so.. It ran cool from then on.. Guess the system needed burped or something.

 

Well, that's one way to do it !

 

Yep I ended up being a combination of an air pocket in the system along with a faulty thermostat. I took the tstat out completely to burp the system.. It made life a lot easier.

I will add to this and say I thought the system was totally burped, but it wasn't. I ended up adding 3.5-4 gallons of coolant total by the time it was all said and done, but I also drained the entire system beforehand..

 

hmm I hope you mean the sending unit not the T-Stat

 

I actually took both out when I burped it. I'm about to make everyone freak with this next sentence. I still haven't put the tstat back in......... That being said I drive it maybe twice a week, about 3.5 miles. I'll eventually put it back in... Just haven't had time to mess with it the last few weeks.