Overheating or normal?

[Thoz43]

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?

 

[Unclewolverine]

Sounds like an air bubble to me.

 

[Thoz43]

Sounds like an air bubble to me.

Said airbubble can be removed how?

 

[Thoz43]

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

 

[75SV1]

The RENIX era coolant reservoir has its issues. I'd convert to a later open reservoir. The red stuff is probably rust in the system. I'd do a flush. I forget what ones I've used. Red Devil and there is another that does well on rust. A bit expensive though.

 

[lawsoncl]

The cap on the pressure tank is a known failure point. The replacement tanks flex and the poorly made caps tends to not hold pressure or work loose, causing a loss of pressure and a boilover. I had one that would open a 6-psi. A cap for a 710 Volvo has much better formed threads and holds pressure correctly. If you replace the radiator, I highly recommend converting to the open setup from a 92(?) or later. Get heater hoses for a 96+ to eliminate the failure prone heater control valve.

When filling, I like to loosen the temp sensor at the back of the head to let the air bubble out. 1/8" hole at the top of the thermostatif it doesn't already have one to let air bleed through also really helps.

 

[75SV1]

Also, some replace just the hose and put in a section with a radiator cap. So, eliminating the pressure bottle.

 

[Thoz43]

The cap on the pressure tank is a known failure point. The replacement tanks flex and the poorly made caps tends to not hold pressure or work loose, causing a loss of pressure and a boilover. I had one that would open a 6-psi. A cap for a 710 Volvo has much better formed threads and holds pressure correctly. If you replace the radiator, I highly recommend converting to the open setup from a 92(?) or later. Get heater hoses for a 96+ to eliminate the failure prone heater control valve.

When filling, I like to loosen the temp sensor at the back of the head to let the air bubble out. 1/8" hole at the top of the thermostatif it doesn't already have one to let air bleed through also really helps.

So it sounds like with my new reservoir, I should purchase the Volvo cap and be good to go? When the Jeep is running, the coolant is bubbling into the reservoir. Is this because of the poorly made cap?