Alright guys, I’ve done research for months now, and I could find nothing that described my issue. My uncle and grandfather, who used to race an XJ were stumped as well, so here I am hoping someone can help me.
The XJ, 1989 Laredo 4.0 with only 84000~ miles on it. Bought it in 2023 on non-op with 74000~ miles and fixed it up and its my daily since. It goes back and forth breaking stuff.
The issue. Overheating. Since owning it, I have replaced the waterpump, thermostat, radiator, fan clutch, and electric fan, and I have cracked 2 separate coolant reservoirs, most previous one about a week ago. Back in Janurary I installed an electric fan switch just to switch it on when I start it up. It ran great for about 5 months. At 75~ mph I would be at about 190-195° and idle at 210°. The other day, I was driving home, and my coolant reservoir cracked and coolant spewed everywhere. That, and along with one of the “T-fittings” which connected my thermostat housing to the top hose on the coolant reservoir. Today I put on new parts and the Jeep idled at 245°~ and at about 40~mph (neighborhood) it was 210 and would surge back up. I had the heater on and it didn’t make a difference. Im in California and today it is about 99° outside. I should probably mention I did not drain the radiator or block of coolant when putting on the new reservoir. What could my problem be?

 

Was the coolant system burped after adding more fluids?

 

How do I burp the closed system?

 

Went outside to check the reservoir level, its empty and has dirt layered on the bottom. Does this indicate anything?

 

I am confused or misinformed. I have never had a coolant reservoir crack from an overheat. I do not think the reservoir is sealed, there should be a outlet tube so that if they do "overfill" the fluid can leave via the outlet. I would check to make sure the outlet tube or the plastic nipple for it are open. I can imagine with cheap parts now days that the inside of the outlet nipple could still be plugged with plastic and need to be opened prior to use. Double check you have the correct PSI cap on the radiator too.

There are some funnel kits that connect to the radiator to help the air burp (just google it).. But anytime I refill a cooling system I always elevate the front end of the vehicle about 6" or so. Every vehicle is different but just doing that should help get the air out. But if you have something else wrong you may have to correct that issue first.

 

And about the dirt in the bottom of the reservoir. That's corrosion from inside the engine and is typical of a vehicle of that age. As long as your don't see "mud" when you pull the rad cap off (cold engine) you'll be ok. I'm assuming you flushed everything really good if you replaced all that stuff.

I once had a 96 mustang with the 3.8L v6 that I had to flush the coolant system on once a week for about a year before it would actually clean out everything and the coolant stay green when I checked. When I bought it the thermostat had dissolved from rust and only a piece of the shell remained. lol.

 

It is a closed system the reservoir does not function the way an open system does. The tank has the pressure cap on it, it is a totally sealed system that does not let air in or out. That is why it is crucial to burp a Renix era closed system.

 

Well that clears that up! Thank you for the info. I've only ever had a '97 and my current '00 XJ.

Now knowing that, I would be doing a sniffer test to make sure the head isn't cracked and getting compression in there..

 

Technically the cap does have a pressure release, so it's still a slight misnomer that it's completely closed. If you're having problems with the cap, get one for a 710 Volvo. Much better fit and won't keep popping loose and boiling over.

Reservior being empty is likely just air working it's way out.

 

If air is working its way out, does that mean I have to keep burping it or do I have a bigger problem somewhere else, such as a bad head gasket?

 

Fill the high pressure overflow tank to the fill line and put the cap back on. Remove the radiator cap and top it off, leave it off, start it up and let it run for a bit, as its running, if there's an air bubble it should bubble out of the radiator fill, keep topping it off as needed with more coolant, once it stops bubbling (burping) put the cap back on.

See if this helps with the over heating issue.

 

I have no radiator cap due to it being the closed system. I only have the reservoir cap.

 

Then leave the reservoir cap off, you just don't want it building pressure as that will hold the bubbles in place

 

This morning I went outside and the reservoir was empty, filled her up with the cap OFF until it warmed up to 210° and it began to overflow. It was still “overheating” to about 245° and I let it run for about 10 more minutes with the cap ON and shut it off for roughly 8-9 hours. Checked the reservoir and it was empty once again so filled it to the “minimum” line and put the cap ON and let it idle till it warmed up. It took about 15-20 minutes to reach 210° and another 3 to go past it around 225°. At that point I drove it around my neighborhood and it ran at 190° and idled 210° all the way up my 1 mile street while in 4low. Tomorrow morning Ill check the reservoir once more but I think it should be good.