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NAXJA Forum User
Broken Arrow, OK
I bought a 1988 xj 4wd 4.0 for my daughter. The temp runs right up to the red. has not spiked into the red yet. It cools back down at highway speeds or at idle, or around town but still runs on the high side of normal.

the upper hose does not get 'hot' but gets pretty warm. might be conducted heat. and when i squeeze the hose i can not feel any water/pressure.

replaced the thermostat yesterday. did not fix the problem.

any ideas?

Sounds like you have a trapped air pocket in the head.

Best thing to do is to pull the upper rad hose and fill from there. Did the replacement t-stat have a bleed hole in it? If not, you can drill a little hole and put it back in with the hole at the top. I'm sure 5-90 can get more specific on this matter.

The radiator hose should be very hot when the motor is completely warmed up. Too hot to hold on to for more than a second or so.

I just did my t-stat tonight. First one I got was sticking open. I drilled an 1/8" hole top and bottom and it self bled. Drill in the flat part of the t-stat, inside the ridges. I did mine at 90* to the mounting point for the pellet. I know it works because I had to pull it a second time to fix a drip. My t-stat housing has something boogered on the mating srface.
In a vehicle this age, the water pump may have an eroded impeller.

When the engine is fully cold, try removing the radiator cap, and while in park, idle the engine until the thermostat opens, looking for plenty of coolant welling up.

A dying water pump wont stir up much coolant.
I drilled a hole in the thermostat. Is that all I have to do to burp the system?

still doing the same thing but maybe not as bad. the hose off the thermostat housing to water bottle gets too hot to hold , but I can still hold the upper radiator hose.

guess I will go get cap tomorrow.
It's that global warming. Causes all kinds of trouble.
Cure is to give all your money to China, or some other place where it will be distributed more equitably.

I dunno it's tough to tell by just the gauge or the feel of the hoses. But it seems to me if it were really overheating, and especially without pressure, it would be boiling.
so can a bad coolant tank cap cause these symptoms?

idles fine
at around town speeds and at highway speeds it does the following:

1) temp gauge pegs up to almost red then comes back down to about 220+
2) coolant does not overflow
3) upper radiator hose gets warm/hot but not too hot to hold
4) upper radiator hose does not feel like there is any pressure/fluid in it.
at this point I think you need to verify that the temp guage is reporting correctly.
was planning on replacing the sending unit...because I thought it might be off on then temperature it was reporting...but with the smaller hose out of the thermostat housing back to the tank getting so hot, I figured the thermostat should have opened and heated the upper radiator hose to a temperature that would make it too hot to hold on to for too long.
If it's still a stock Blackstone rad, then it's time for a new one. The crossflows clog up quciker than earlier vertical flows. When you do so, put in a '91 rad so you will have an accessible cap to fill it with. If you have air trapped, then loosening the temp sensor (behind the throttle body in the head) when the motor is cold will let it burp. Jeep factory gauges are for amusement purposes only!

replaced the coolant sending unit and in the process burped the system. Put on a new coolant cap at the same time.

So far it has ran right at about 210 by the gauge. I know the gauges are not accurate and for 'entertainment purposes only' on these 'ol jeeps.

we'll see how it goes and if problem returns I will post update.

Thanks for all the input everyone
Your clutch fan might be out. They typically don't last as long as yours. The viscous fluid breaks down in the clutch, it seems like it's working when you look at it, but at running rpm's, it's not spinning. They cost something like $40 at pep boys and take 30 minutes to replace.
Replacing the fan clutch is a good suggestion, and you are correct in assuming that your factory gauge is inaccurate. I just went through a simmilar situation with my XJ, and after throwing a few parts at it,cap,thermostat, etc,I decided to install a quality aftermarket temp gauge, turns out my factory gauge was reading 20 degrees over the actual temp.