"gogmorgo - rowing gears in a Grand Cherokee" (gogmorgo)
08/07/2018 at 23:52 • Filed to: None
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"gogmorgo - rowing gears in a Grand Cherokee" (gogmorgo)
08/07/2018 at 23:52 • Filed to: None | 0
| 14 |
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.
LOREM IPSUM
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 00:56 |
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Pressuring the system raises the boiling point of the coolant, allowing the system to run at higher “normal” operating temperature, thus providing better emissions for some reason that isn’t entirely clear to me.
Chances are you just need a new thermostat.
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gogmorgo - rowing gears in a Grand Cherokee
> LOREM IPSUM
08/08/2018 at 01:15 |
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Yeah. But neither system offers any inherent advantage in pressurization. The closed system extends the pressure to the reservoir, but an “open” system still retains pressure , just not in the overflow.
Specific running temperatures allow more precise engine chemistry, which is good for emissions control. Higher pressure only resists boiling, which is advantageous in that steam doesn’t provide very good heat transfer.
I fail to see how the thermostat has anything to do with my issue. It’s it stuck closed, otherwise I would’ve been massively overheating. Sticking open wouldn’t cause any ill effects other than slow warmup.
Longtime Lurker
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 02:30 |
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I couldn’t remember what year St. Jimmy is but this should still be relevant.
#13 is the hose you talk about coming from the rad cap the pressure relief hose
#19 off the reservoir should be the return hose the engine pulls coolant from.
Of course I could be completely wrong as I don't know what I'm talking about.
RedLeader289
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 07:29 |
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“I don’t really see any way for that flow to reverse in any sort of capacity.”
It’s all about pressure in the system. When your “open” system car is shut off and cools down the pressure drops in the system. IF as a result of that pressure drop a void is formed within the system a vacuum is formed (because hydraulics) that sucks coolant from the overflow tank back into the system. In order for this to work you have to have a proper functioning radiator cap (because if the cap is faulty then it isn’t holding pressure in the system).
It works. Trust me.
If you don’t think so then drain a quart or two out of your radiator and go for a drive, then shut it off and 30 mins to an hour later your overflow will be empty.
BigBlock440
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 07:33 |
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Radiator caps have a valve in the center with another spring. When the system cools, stuff shrinks back down creating a vacuum, sucking the coolant back in.
If only EssExTee could be so grossly incandescent
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 08:28 |
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My BMW has a closed system and it’s a bitch and a half to get filled and bled and whatnot, but once it’s done it seems to work perfectly. It does run under supe r high pressure though, I once blew off a (plastic) bleeder screw doing some moun tain driving. If you do get a leak you have to wait for the engine to cool off completely due to the live nature of the reservoir.
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LOREM IPSUM
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 08:29 |
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Had a mazda that was doing something similar. I’d park it, and it would make glugging noises and puke coolant out of the overflow when parked. I think the t-stat was getting lazy because I’d notice temp fluctuations while driving. W hen the engine was shut down and fully heat soaked, it may have been hot enough for the t-stat to be open, but since it was lazy it wasn’t , the system pressure would build as a result and push the expanding fluid out of the overflow tube.
Driver_41
> LOREM IPSUM
08/08/2018 at 08:48 |
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Higher operating temperatures are more efficient. If less heat from combustion is lost to the atmosphere, that heat can be used to do more work on the piston.
crowmolly
> BigBlock440
08/08/2018 at 09:22 |
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This is also why “puke tanks” aren’t good for anything except catching overflow. You can still end up with low coolant with one of those.
DipodomysDeserti
> gogmorgo - rowing gears in a Grand Cherokee
08/08/2018 at 12:52 |
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A vacuum is created when vapor passes from the radiator to the overflow bottle. This vacuum causes coolant to be sucked back into the radiator before the cap closes. I know it works because I have a leaky thermostat gasket on my Jeep and have to fill up my coolant resevoir every few months.
just-a-scratch
> If only EssExTee could be so grossly incandescent
08/08/2018 at 14:50 |
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Have done. Can confirm.
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If only EssExTee could be so grossly incandescent
> just-a-scratch
08/08/2018 at 16:06 |
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They sell brass bleeder screws on Amazon. Anyone with a 90's or earl y 2000 's BMW should buy a set as preventative maintenance.
Die-Trying
> crowmolly
08/08/2018 at 16:52 |
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YES....... unless you can cheat. i try find ways of mounting the puke tank( and i usually use an old rv motorhome puke tank, or old lawnmower gas tank) above the radiator, so that the tank is always trying to drain itself back into the radiator. you just have to find one that holds just enough puke.........
old junk trucks usually have plenty of room to figure the stuff out.....
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gogmorgo - rowing gears in a Grand Cherokee
> Longtime Lurker
08/11/2018 at 12:58 |
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19 is just the overflow hose from the reservoir. When the level in the reservoir gets too high it just dumps on the ground via that hose.