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Coolant overflow bottle question
- Thread starter yardape
- Start date
5-90
NAXJA Forum User
- Location
- Hammerspace
On the typical "open" system, the radiator cap actually has two valves in it...
One is the "pressure relief valve" (PRV) that works to keep the cooling system at or near specified pressure. When you see the "pounds" rating on the cap (typically 6/13/15/16,) that's the pressure that the cap is rated to hold, in psig (pounds per square inch, gage - or, pounds above atmospheric pressure.)
The other is the "vacuum relief valve" (VRV) - which is what allows overflow coolant to be drawn back into the system. You'll probably note that the overflow hose is connected at the bottom of the overflow tank - there's a reason for this.
As most things (like coolant) are heated, they tend to expand. The point of the overflow tank is to chatch this expanded coolant for re-use. Coolant in the overflow tank, above and beyond the small amount that is specified to go in there for a system fill, is coolant that expanded past the PRV, and therefore went down the overflow hose into the overflow catch tank.
When the system cools down, the coolant again contracts, which creates an air space above the coolant in the radiator. Since no air is allowed in there, you have a relative vacuum between the radiator and the ambient air pressure - this is where the VRV comes in.
Once a vacuum is set up in the radiator, the VRV opens, and coolant is passed through the VRV back into the radiator, impelled by atmospheric pressure (to be technical, it isn't "sucked" into the radiator, it's "pushed" into the radiator. Such is the nature of relative vacuum.)
When you check a typical radiator cap, you'll note that there is a large flat rubber gasket and a silver metal "thingy" in the middle - that's the VRV.
You test the pressure seal with a pressure tester, and/or visually inspect the rubber gasket.
You then take a pointed tool (usually a knife tip) and GENTLY pry up under the silver disc to make sure it opens easily. It's pressed closed by system pressure in normal operation, and should open readily when you pry on it anywhere around its edge. DON'T PRY TOO HARD OR TOO FAR - or you'll wreck the cap and need another one. Usually, just the fact that I can get a knife point in there easily and it opens is enough for it to pass - you shouldn't notice any pressure needed against the cap, and there is a VERY light spring in there to hold it shut until pressure bears against it.
Make some sense now? If not, go get a radiator cap and re-read this - it will.
5-90
One is the "pressure relief valve" (PRV) that works to keep the cooling system at or near specified pressure. When you see the "pounds" rating on the cap (typically 6/13/15/16,) that's the pressure that the cap is rated to hold, in psig (pounds per square inch, gage - or, pounds above atmospheric pressure.)
The other is the "vacuum relief valve" (VRV) - which is what allows overflow coolant to be drawn back into the system. You'll probably note that the overflow hose is connected at the bottom of the overflow tank - there's a reason for this.
As most things (like coolant) are heated, they tend to expand. The point of the overflow tank is to chatch this expanded coolant for re-use. Coolant in the overflow tank, above and beyond the small amount that is specified to go in there for a system fill, is coolant that expanded past the PRV, and therefore went down the overflow hose into the overflow catch tank.
When the system cools down, the coolant again contracts, which creates an air space above the coolant in the radiator. Since no air is allowed in there, you have a relative vacuum between the radiator and the ambient air pressure - this is where the VRV comes in.
Once a vacuum is set up in the radiator, the VRV opens, and coolant is passed through the VRV back into the radiator, impelled by atmospheric pressure (to be technical, it isn't "sucked" into the radiator, it's "pushed" into the radiator. Such is the nature of relative vacuum.)
When you check a typical radiator cap, you'll note that there is a large flat rubber gasket and a silver metal "thingy" in the middle - that's the VRV.
You test the pressure seal with a pressure tester, and/or visually inspect the rubber gasket.
You then take a pointed tool (usually a knife tip) and GENTLY pry up under the silver disc to make sure it opens easily. It's pressed closed by system pressure in normal operation, and should open readily when you pry on it anywhere around its edge. DON'T PRY TOO HARD OR TOO FAR - or you'll wreck the cap and need another one. Usually, just the fact that I can get a knife point in there easily and it opens is enough for it to pass - you shouldn't notice any pressure needed against the cap, and there is a VERY light spring in there to hold it shut until pressure bears against it.
Make some sense now? If not, go get a radiator cap and re-read this - it will.
5-90
Blaine B.
NAXJA Forum User
- Location
- Northwest Indiana
I don't think that's what he's getting at......I thought this too, once.
The hose attaches to the top of the resivoir. You wonder how it sucks coolant up, right?
Well, what the rubber hose attaches to on the resivoir, is actually a pipe that runs on the inside of the resivoir to the very bottom of the resivoir.
It sucks coolant from that point, as well as pushes coolant out from the radiator into the resivoir.
Is that what you were asking?
The hose attaches to the top of the resivoir. You wonder how it sucks coolant up, right?
Well, what the rubber hose attaches to on the resivoir, is actually a pipe that runs on the inside of the resivoir to the very bottom of the resivoir.
It sucks coolant from that point, as well as pushes coolant out from the radiator into the resivoir.
Is that what you were asking?
5-90
NAXJA Forum User
- Location
- Hammerspace
yardape said:
"The crux of the bisciut/is the apostrophe(')."
Check carefully - sometimes they go right in at the bottom, sometimes they go into the side with a feeder tupe, and sometimes it actually goes into the cap, with an "extension line" going from a nipple on the bottom of the cap down to the bottom of the bottle.
Either that, or there's two nipples - one top and one bottom - and you've probably got them reversed. The "top exit" is for when the overflow overflows, and it allows you to direct that overflow to a convenient location...
5-90
Blaine B.
NAXJA Forum User
- Location
- Northwest Indiana
Newer cars generally have the hose built into the cap which reaches down to the bottom of the resivoir. Older cars have more complicated systems. On the XJ's the hose enters at the top, and then has an internal molded passageway that reaches to the bottom of the resivoir....
On an old 1980 Oldsmobile we have, the rubber hose connects directly to a nipple at the bottom of the resivoir....
It all depends, but either way, it works.
On an old 1980 Oldsmobile we have, the rubber hose connects directly to a nipple at the bottom of the resivoir....
It all depends, but either way, it works.
Blaine B.
NAXJA Forum User
- Location
- Northwest Indiana
5-90 should put that as a sticky, after typing all of that out!
5-90
NAXJA Forum User
- Location
- Hammerspace
S'OK - not only do I have it in my Archives (tapping forehead...) but I'm also putting it into the Cooling System FAQ I've got somewhere - and need to get back to work on one of these days...
5-90
5-90