dola52

I'm starting a new thread about this instead of reviving a dead one. 1998 Cherokee 4.0L. 245K miles. My daughter's Jeep. Problem started when the water pump crapped itself and dumped all the coolant while she was driving. It overheated of course, to the point that I at first thought it might have toasted the engine, but seems to be ok other than this temp spiking thing. I replaced the radiator, thermostat, cap, all hoses, and flushed it last year. After the water pump crapped itself, I replaced it, flushed it again, and went ahead and threw a new thermostat in. I burped it as well as I could. It seemed to be fine at first, but after it gets good and hot, temp around 210, it will suddenly spike up to about 250 and gauge light comes on. After a few minutes it will just as suddenly drop back to 210. At first I thought it may be a bad temp sensor, so I replaced that, to no avail. I thought that maybe the temp sensor wiring harness was shorting out, so while it was spiked, I unplugged the temp sensor and plugged the plug into the old sensor, which causes the gauge to drop back down and will respond to heat added to the sensor while hanging. When I plugged it back into the installed sensor, pops right back up to 250, so to me that rules out a short. I burped it some more, even loosening up the temp sensor to release any air behind it. I finally decided to get a 180 thermostat and see if that would help. Did run a little cooler until it spiked to 250, so didn't help. I finally just pulled the thermostat out altogether, and it doesn't spike without a thermostat. It would slowly make its way up to 200 - 220 and stay in that range, even after prolonged idling. I'm thinking it has to be air in the system, but I've really burped all I can get to come out. The overflow tank is at the full mark, and doesn't fluctuate more than in inch between warm ups and cool downs. Now, I've found several threads, on this one and other forums, with people having the same problem, but no one ever comes back and posts a final verdict. After sitting and thinking about it, the only thing I can think of, the replacement thermostats I used have no burp hole. What is the possibility of that letting air get trapped in the thermostat housing right where the sensor is? I know its a far reach, but I cant figure out what else could cause this. I don't even know if an air pocket at the sensor would cause it to register a temp spike. Anyone have any ideas about this? I'm just going to run it with no thermostat for the time being, but winter will be here before long, she'll need a thermostat in it by then. I may just go ahead and either find a thermostat with a burp hole, or drill a 1/8" hole in what I've got. If i figure it out, i will post my findings, but any good advice would be appreciated.

lawsoncl

Try replacing the radiator cap? Could be it's failed and not holding pressure.

Roler

Personally, the thermostat is the only thing I've ever gotten from the dealer.

Did the coolant in the overflow bubble during those spikes?

5-Speed

Sounds like there is still air in the system to me. Did you fill up the overflow with any coolant? It needs some. One you get the engine up to temp, you should be able to shut it off, wait a couple minutes and see it suck fluid from the overflow back into the engine. Repeat this cycle until it stops sucking back in fluid once shut off.

ROADKILLJEEPXJ

I'm having a similar issue my water pump was leaking and I replaced it and the gasket and a little after that I had a temp spiking problem. Replaced the thermostat didn't help. I have a tool from Snap-On that sucks all the air out of the cooling system which then I connect the jug of coolant and it pumps it into the system, since I've done this that verifies I have no leaks no air pockets and that I was full of coolant at time of thermostat install. After driving around I have noticed coolant leaking onto the hose underneath the radiator cap. Replaced the cap and I've still got a leak. Actually have put 2 16ibs caps and 1 18ibs cap. None of the caps helped and finally it overheated to the point the system was boiling and had to pull over. Got it to the shop and performed a head gasket leak test whish thankfully I did not crack a head. I almost feel as if I have a defective radiator. The previous owner had just replaced it before I bought it so I kind of just didn't consider it. Now that I think about it in my mind the system is designed to operate at 16ibs of pressure which it is unable to due to the cap not sealing. I feel like a new radiator will fix my overheating issue however it could be a waste of $127 idk what to do lol.

lawsoncl

You could borrow the pressure tester from one of the auto stores, pump up the radiator and see if it leaks at the neck or the hoses. Pulling a vacuum might seal a leak at the hose connections. You can also test the radiator cap.

ROADKILLJEEPXJ

Went into the shop today to try out what you said. Grabbed my pressure tester and tested the cap and it help 18Ibs solid. Tested the radiator and figured I'd probably have a leak where the cap seals up, turns out I do not? I'm stumped on this one. I'm going to fill it up and drive it and double check both of my radiator hoses are getting pressurized. If Dola52 ever sees this are you having any similar issues with your radiator or cap? This was just the closest thread to my current problem.

awg

The XJ is a bit of a mongrel to bleed cooling system (like many vehicles)

This is what I do, works almost every time in one go

Park vehicle nose up on a reasonably steep slope, fill cooling system, idle engine with rad cap off till thermo is open, pour in as much coolant as possible. Ensure overflow resevoir is topped up

Leaving cap off, go back inside and have dinner. Next day, start engine and idle till thermo is open, still no cap, add any coolant that you can (air bubbles rise overnight and exit rad neck)

Leave engine idling with cap on for a few mins..temp should stabilise with electric fan switch on (in warm weather).

Test drive, since I have been doing this, I cannot recall any issues

XJ are known to get air pockets, and it happened to me at least once

Have had a dodgy cap, a spare is worth having/trying

lawsoncl

I just loosen the temp sensor at the back of the head to let the air out. Also use a thermostat with the bleed hole.

dola52

Sounds very similar to the problem I'm experiencing. Same with radiator cap.

lawsoncl

Nothing stopping you from drilling a 1/8" bleed hole in a thermostat that doesn't have one, btw.

lawsoncl

Have you done a block test, and ruled out a head gasket issue? I had an Equinox that would act the same way until I figured it out. It would randomly spike the temperature because it was pushing exhaust into coolant and forming an air bubble in the head.

awg

I dont like doing that because I broke one once, and that was enough for me, not cheap out here mate, and on my 96 its awkward to get at

if the OP suspects a head gasket leak, almost always, when the slow bleed method is done, and you finally top up the rad to the neck, there should be NO bubbles

If there is bubbles means exhaust gas or excessive heat is entering system

ROADKILLJEEPXJ

It turned out to be my electric fan, A/C fan whatever you want to call it. Every time I worked on it the fan would always spin but it would be slow I kind of ignored it. Got inside to check temps realized a/c was running and seen the fan was not running. Maybe you should try the same the part I purchased was $80.

ROADKILLJEEPXJ

If you do order the part I used it doesn’t come with a plug just 2 wires so you’ll have to cut the plug off the jeep (I left extra wire on mine incase I’d need it in the future.)

You could either solder the connections which makes it a chore to remove in the future. Or what I did use posi locks. I think that’s what there called anyway I’ll show pictures of both. Hope this helps