Getting Discouraged with my MJ (radiator issues)

[Pete M]

anyone know what pressure the 87-90 radiators are rated for? 

[gogmorgo]

It still shouldn’t be building enough pressure to pop the thing so long as that rad cap is working. Rockauto sells both 15 and 16psi caps as OEM replacements. The popular Volvo cap is rated at 22. But again, unless the overflow line is pinched or plugged, or the cap isn’t opening until way higher pressure than it should, it’s not making enough pressure to blow anything up. 

[Classy Comanche]

Now I'm very confused. How full should the metal tank be? Half or full to the ring? Everyone initially was saying full all the way because the thing is is my radiators keep failing. Not my tank. In theory if I have super high pressure wouldn't my aluminum tank would have blow up? It does spit coolant into the tank. That's for sure. My thing is where should I fill it to. Right now it is full. The catch tank is almost empty as of now

[Pete M]

does the tank have any hose coming up into the bottom of it?

[Classy Comanche]

@Pete Mit indeed does. I'll go snap some pictures, give me a few minutes 

[Classy Comanche]

Here ya go @Pete Msorry it's dark. I work 7 to 7

[Pete M]

So there's a new tank... plumbed exactly like the old tank...  sure sounds like a closed system to me.   

 

 

[Classy Comanche]

Alright so what I'll do is have my radiator changed Monday since my car is finally done. I'll drop it off and tell the guy working on it to fill it a little above half in the tank and leave the plastic one with a tad in it? I figured this would have been a open system? He says it is and others have too and others have also said no also. So I'm sorta at a loss. I'll try about half coolant next time

[Pete M]

don't take my word for it.  wait for some more opinions to roll in.  wouldn't be the first time something escaped my brain.    but I installed a late model radiator (has a rad cap) on my 88.  that didn't automatically make it an open system.  and my rad cap has never popped open. 

[Pete M]

since this has gotten pretty tech-heavy, I'm going to move it.   maybe more eyes will see it then. 

[Classy Comanche]

Thank you sir! I do appreciate it. 

 

https://www.cherokeeforum.com/f2/question-those-you-macs-radiator-shop-coolant-tank-renix-212247/

 

 

 

Found this link. Perhaps I should indeed go about halfway?

[Classy Comanche]

Wait! Would my OEM cap from my old slightly cracked plastic bottle work on this? I fill it halfway leave the overflow tank and put on the old plastic nipple cap from the old bottle?

[Pete M]

if I remember right, the plastic cap is a very different fitment method.

 

[gogmorgo]

The metal tank. What’s the pressure rating on the cap? 
 

The pressure through the entire cooling system, or at least the pressurized parts of it, will be pretty well even throughout. 
 

A closed cooling system is one where the entire system pressurizes. Imagine a juice box. Don’t poke the straw in just yet. Squeeze the box. Pressure builds inside, if you squeeze hard enough it’ll pop the little foil circle for the straw open to blow off the excess pressure, and for the sake of the metaphor we’ll say the little foil circle closes and seals again once the pressure drops. All the juice runs out onto the ground, but the system is happy because it’s no longer under excess pressure.  So when it cools down and heats up again, it won’t build an excess amount of pressure, and the juice box stays closed and sealed, nothing goes in or out.
An open cooling system is exactly the same, but now you’ve got a straw shoved into it. The other end of the straw is in a glass of juice. When you squeeze the juice box, pressure builds but now instead of popping the lid and dumping juice on the ground, it pushes it out through the straw into the glass. The level in the glass goes up. Then you let go of the juice box, and for the sake of the metaphor, we’ll say the juice box returns to its original shape, so it sucks the juice back in, pulling it back through the straw from the glass.

 

The confusion I think we’re seeing here whether it’s open or closed is that what amounts to an open conversion was done without using the later parts that would be typical of an open system conversion. Instead of replacing the radiator with a later one with a cap, it just had a replacement reservoir put in with an open style radiator cap on it. This is advantageous because you won’t need to replace the radiator or reroute any of the factory radiator or heater hoses. This would be the easiest and likely cheapest way to covert the system if you had an old perished coolant reservoir that didn’t hold the pressure. The larger coolant volume it allows for will help keep temperatures more stable as well. 

Ultimately it makes no difference if you’re doing the fluid exchange from the top of the radiator or from the reservoir that at this point is essentially just a fat heater hose that collects coolant. Remember the pressure is the same through all the cooling system, so it makes no difference where you’ve got your cap on the system. 

I would say fill the metal reservoir to the top, and then make sure there’s some in the plastic reservoir. But unless the system is overheating, don’t bother adding coolant anywhere if you’re changing out the radiator on Monday, cause it’ll just all be getting dumped out anyhow. 

[Classy Comanche]

Will do! It cools, just frustrated with the radiator lol. So the cap I got is the one that came with tank. 16lbs of pressure. That's a good metaphor too. I have never thought of it that way

[Classy Comanche]

@Pete Myou asked about the pressure rating, I did too earlier on. Found this on naxja. Nothing like patrolling through 18 year old threads finding good info! I believe the Cherokee and Comanche run the same stuff. Just different body types with one a truck and other SUV

 

https://www.naxja.org/forum/showthread.php?t=59091

[PIKE]

Your making this too complicated.

 

MAC Tank + Expansion bottle =  Open System

 

Its that easy. Your mechanic has it together correctly.  Mac tank should be full.

[Classy Comanche]

Awsome! I'm glad that is correct. I just get all kinds of confused when like I'm told yes and no it is or isn't a open system. From what I have deciphered how it works is the Mac tank is full and has the engine heats it pulls from that tank and builds pressure then spits back what it doesn't need in the catch tank then when there's room it'll take it back in

[PIKE]

Now you are getting me confused, haha.

 

The Mac tank has a open system radiator cap. Think of it as an extension of your radiator, that's why the Mac tank should be full. The small hose that comes off the tank under the radiator cap should go to the bottom of an expansion bottle.

When your motor heats up fluid expands into the bottle, during cool down it gets sucked back into the Mac tank.

Your expansion bottle should have fluid in it too, whether hot or cold. Half full would be a good place to start.

[Classy Comanche]

Oh that's how it's been but the radiators leaking on me has been plaguing it. I'll continue to keep it that way

[Pete M]

so what system do I have?  everything is stock 88 but the radiator has a cap. 

[fiatslug87]

Open system 

[gogmorgo]

Unless the rad cap is set up to move fluid in and out of an expansion bottle it's a closed system still.

But it's more a difference in semantics than in function or configuration.

Either system needs to retain up to the maximum pressure and adequate coolant volume on the pressurized side with some facility for expansion.

You could even set up a cooling system with an expansion tank to act as a catch can when the pressure relief pops off instead of venting excess pressure straight to atmo/ground, and it would still be closed if it's not pushing pressure in/out during normal function. All you'd need to do that with a typical open system is install a higher pressure rad cap to stop it from popping open unless it gets real hot. Jeep themselves may even have been trying to do that in the ZJ when they went from a 16 to 18 psi cap without any major differences in the cooling system. Again, it's more of a semantic difference than one of actual function or even configuration.

 

But we're getting in the weeds for no reason here. The way buddy's Jeep is set up here seems to work just fine as long as it's containing its volume of coolant, which it will once the cracked radiator is dealt with.

[Pete M]

25 minutes ago, fiatslug87 said:

Open system 

 

my 16# cap has never popped.   

 

does fluid regularly flow in/out of the cap on that macs tank? 

 

it may be semantics, but I feel like I need to know what makes what before I make suggestions to other owners.   

[gogmorgo]

It may be an important discussion to have, I just don't think we need to be confusing anyone on this particular thread. There's a lot of overthinking going on already.

 

Probably the best way to break it down for our purpose is how the system deals with expansion due to heat.

A closed system has a pressurized reservoir with air at the top, and as the system heats up it compresses the air to deal with excess coolant volume. As it cools the coolant shrinks and the air expands.

An open system doesn't leave an air gap. The expanding coolant pushes itself into an overflow bottle, and sucks back in as it cools and shrinks.

There are many ways to configure the system, but unless you change that one thing, you haven't changed from closed to open or vice versa.

 

 

The replacement caps listed everywhere are usually 15-16 psi. Would make sense a 16psi rad cap wouldn't ever open if the system itself isn't ever seeing more than that. Also the cap itself doesn't blow off, it would just be a release of pressure out the overflow. And if there was a small pocket of air in the filler neck, it would just release that, coolant wouldn't necessarily come out. If the overflow line is capped off securely enough nothing would ever come out of it. Any extra pressure held on the open side of the cap will also help to hold the cap closed, increasing its ability to hold pressure beyond it's rating. I don't know when the upper seal on the cap might fail and start puking out coolant, but I imagine it to be high.

 

On 2/22/2023 at 1:10 PM, Classy Comanche said:

Yeah I got a plastic tank attached to my aluminum Mac tank. It does push coolant into there and keeps the aluminum one full

Sounds an awful lot like an open system to me.

Once the rad is dealt with, if you leave some space in the top of the reservoir when it's cold and it fills itself back up after you run it to temp and it cools down, we'd know for sure.