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?

 

[Saudade]

I use the Volvo cap on my '88. Can't say that will fix your overheating issue but it won't hurt. Trapped air can be problematic to work out of the closed system. Takes several heat/cool cycles to purge the air out. Lawsoncl listed some things to help.

As far as the debris in the bottle, it might depend on what it is. If it's rust, a good flush (or two) will likely get the bulk of it out. If it's tiny pieces of black stuff, it could be the inside of the hoses. You didn't say if you changed all of them.

How long was it no-op? When you changed the thermostat and water pump, did you notice how much rust was in the block?

 

[Thoz43]

I use the Volvo cap on my '88. Can't say that will fix your overheating issue but it won't hurt. Trapped air can be problematic to work out of the closed system. Takes several heat/cool cycles to purge the air out. Lawsoncl listed some things to help.

As far as the debris in the bottle, it might depend on what it is. If it's rust, a good flush (or two) will likely get the bulk of it out. If it's tiny pieces of black stuff, it could be the inside of the hoses. You didn't say if you changed all of them.

How long was it no-op? When you changed the thermostat and water pump, did you notice how much rust was in the block?

It was non-op for I think 4 years? I could definitely be wrong. I did change the radiator hoses when I replaced the radiator. There was no rust in the block what so ever.

I think at this point I need to just flush it all out once, replace the cap and try to eliminate all the air bubbles I have.

Would it be a terrible idea to eliminate the thermostat in general? Theoretically it would allow air out of the system and it would no longer be building pressure until it reaches a certain temperature right?

One of two things happened, I have air or a blown head gasket and I really hope its air.

 

[Saudade]

In general? You mean permanently? I think that's a bad idea. For the duration of the flush? I think that's NOT a bad idea. Even when fully open, thermostat can still block enough flow to trap some crud. If you use a chemical flush to ensure you get any remaining scale, rust or other buildup out, follow up with plain water to rinse any remaining chemical out. Then drain again, flush components with the hose, then seal up and refill with coolant.

I "prefill" my radiator before connecting the upper hose (all others hoses connected). I used to use about a 1ft piece of hose on the end of a funnel. The other end goes into the upper rad port (like a gooseneck) so I can fill the rad until it starts to pour out the port. Then attach the upper hose and continue filling through the bottle. Last time I changed the hoses, I cut the end off the old upper hose with enough of the bend to orient upwards. No longer need the small funnel and it fills much faster due to the larger hose diameter.

Even with this method, it takes me 2 or 3 heat cycles to feel confident I have all of the air out. With the engine running, I can see the coolant return to the bottle. I will see some small bubbles but sometimes that's just the returning coolant churning up the coolant already there. Once I shut down, if I still see bubbles coming from the outlet hose, I'll let it completely cool, check the level, top off if needed, and repeat the cycle. By top off, I mean the bottle about half full. If you look down in the fill port, you'll see a post with a notch. Top of the post if FULL, level with the bottom of the notch is ADD. This is with engine off, cold on level ground.

 

[Saudade]

Also, if you haven't come across it yet, check out Cruiser54's site specfically for Renix.

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