So Oppo. This is something that’s sorta bugged me for quite some time.

I noticed this odd phenomenon yesterday. Saint Jimmy was doing great. All gauges were showing good, etc. Then I cut my drive short after my squishy brake pedal turned into a brake warning light. Yeah there’s a leak about where the hardline connects to the rear axle flex line. Not end of the world, it’ll get fixed. Tradeoffs for open rear line is epic burnouts vs the POS trying to kill me if the ABS triggers, which I learned in the wayback. But I digress.

So. I pop the hood, brake reservoir is effectively empty, I grab a bottle off the shelf and when I come back, the coolant in the overflow bottle has started boiling. Actual boiling or just pushing gas out? I dunno, but it went on for a couple minutes, then quit immediately after I started the engine out of curiosity, and then started back up again about twenty seconds after shutting it back off. Weird, right? I walked away for a bit until it stopped, then gingerly opened the rad cap. Usually I expect a little puff of outward pressure, but I’m pretty sure I was releasing a vacuum opening it. It was about a litre and a half low. I topped it up, and it seems okay now.

But this gets me to my main point. Open vs. closed cooling systems. It’s a somewhat common topic on the Jeep forums, specifically MJ/XJ stuff. Renix era Jeeps used a closed system, then the Chrysler “high output” refresh in ‘91 went to an open system.

What’s the difference? They both function effectively the same way. You’ve got all your cooling hoses, radiator, and reservoir. A closed system pumps coolant through the reservoir, so the reservoir is pressurized. A radiator-type cap on the reservoir vents excess pressure straight to atmosphere.


An open system doesn’t pump coolant through the reservoir. Instead, the reservoir is an “overflow”. Excess pressure from the radiator cap is vented through the coolant reservoir, which is not pressurized, but open to atmosphere.

Is there a particular advantage to either system? Tough to say. After observing a bunch of different examples, I would lean to closed systems being more effective. Having the reservoir as part of the system allows a higher heat capacity within the system, vs the open system, where a portion of the coolant isn’t really doing much. On the Jeep forums though, the closed system is generally scorned as unreliable and less effective. I suspect this is largely due to difficulties in totally filling the system, as there’s frequently no direct fill port for the radiator. In addition, Renix era Jeeps are getting into their 30's now, so the plastic coolant bottles are getting brittle from heat cycling, seals are failing, etc, so they may no longer be holding full pressure, and lower pressure means a lower boiling point. The closed system also appears less tolerant to leaks, at least in as much as it’ll pump out a near total volume of coolant. In contrast, the open system supposedly pulls coolant back from the overflow to replenish itself.

But that last bit is what I get hung up on. I’m not convinced that’s a thing. Sure, the owners manuals, etc, tell you to never touch the rad cap, always add coolant to the overflow, keep an eye on the level there, whatever. But while I agree, the system does push excess coolant and burps air through the overflow, I don’t really see any way for that flow to reverse in any sort of capacity.


The way a traditional radiator cap works, is there’s a spring pushing against what is effectively a valve, holding it closed. The pressure in the cooling system builds against the force of the spring, eventually pushing the valve open, and excess volume of coolant, air, steam, whatever, gets squeezed out. In a open system, the pressure and excess fluids are directed into the reservoir. But then, as soon as the pressure drops again, the valve snaps back closed. The valve, now closed, isn’t going to allow fluid back into the radiator. It just isn’t going to happen. But hapless car owners are going to keep staring at that level in the reservoir that never changes, while the coolant in the actuall cooling part of the system could simply keep dropping, unbeknownst. The coolant in the reservoir, then, would only really serve to reduce the temperature of anything that came out of the radiator under pressure, preventing steam scalding... or something. The Saint was running completely fine all summer last year with a completely dry reservoir.

As to the Saint’s issue, I’m pretty sure there’s just a slow coolant leak. Just gotta find it. Low coolant leads to air trapped which expands a bunch, possibly leading to some excess heating in spots and boiling. I dunno. It’s weird. I’ll figure it out.