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I probably should just leave it alone, but I noticed that the coolant overflow tank does not operate the way that it should. The overflow tank does not syphon fluid back into the system.
I have been searching the forum and diagnosing trying to see what the problem might be. The car works fine. Does not overheat, no lights, no oil in the coolant, no coolant in the oil. I burped the system and bled the system, no change. I made sure the overflow tube was not clogged. I replaced the rad cap. I pressure checked the system and its holding pressure fine, no leaks. Thermostat appears to be working ok, it opens about 190ish. I did a head gasket test by checking for exhaust in the coolant. I did this 4 times at all different RPMs and temps, just to be sure. That all came back negative. At this juncture I am left scratching my head.
What is the proper coolant level for the pressure tank? Whenever I top it off the level is not up to the neck, the lower air bleed valve hose is always exposed,. I will fill it up but then the coolant that I just used to fill it will end up in the overflow tank once it has warmed up. Once I open the surge tank the level is back to where it started. Is this normal? I keep reading things about head gasket but I do not see any signs of head gasket, before I start thinking about the worst possible outcome and pulling everything apart and throwing money at it I want to know if there is anything I may be overlooking? Does anybody have the same issues? I am going to do a coolant flush/refill and thermostat replacement once I move from my apartment into my house being built and I actually have a hose to use. I am just trying to figure out why the coolant wont go back into the system, maybe I should just leave it alone....
mine does the same thing but I know my overflow hose at the surge tank is jb welded on and may not be holding 100 percent.
All of the hoses are tight, I removed the overflow cleaned it out and checked the neck. Its fine. I actually DID apply some JB weld to the neck of the overflow just to be sure! The overflow tank cap cardboard gasket was brittle and dying so I replaced it with a foam one. I am not sure why the gasket matters because the overflow tank cap has vents on the top of it anyway? I ran some compressed air through the hose from the surge tank to the overflow and everything appeared to be normal. How did you confirm where the hose was leaking? Would that hose to the overflow tank leak with the pressure test?
A radiator pressure test won't tell you anything about the overflow system. The tester seals against the lower lip of the cap opening. The overflow is above that.
It sounds like everything is working while the coolant is hot. The coolant is expanding and the cap is relieving pressure by allowing coolant to flow to the overflow tank but as the engine cools the system should draw from the overflow until the pressure drops enough to close the cap to expansion tank seal. So a few possibilities.
1) a leak in the coolant return tubing is allowing the system to draw air.
2) Wrong/ defective cap allowing system to seal before it can pull the coolant back
3) a leak somewhere in the pressurized system which is allowing air in during cooldown such that it eliminates the vacuum state before the coolant can be pulled from overflow.
4) There is enough air circulating around in the system to prevent draw back during cooldown, but the system should alleviate this over time.
If you have a vacuum tester, connect it to the overflow tank down tube, will it hold a vacuum? If not figure out why?
BTW, the overflow tank is vented to allow air in and out of the tank so that it does not become part of the pressurized system. Also by letting air into the tank above the coolant it allows the coolant to be drawn back without vapor locking the overflow tank.
A radiator pressure test won't tell you anything about the overflow system. The tester seals against the lower lip of the cap opening. The overflow is above that.
4) There is enough air circulating around in the system to prevent draw back during cooldown, but the system should alleviate this over time.
If you have a vacuum tester, connect it to the overflow tank down tube, will it hold a vacuum? If not figure out why?
BTW, the overflow tank is vented to allow air in and out of the tank so that it does not become part of the pressurized system. Also by letting air into the tank above the coolant it allows the coolant to be drawn back without vapor locking the overflow tank.
I get that the overflow tank is vented for this reason, so the cap gasket/seal should not matter correct?
I do not have a vacuum tester, I have been renting parts from O'reilly. I will try and see if the lower hose to the overflow will hold vacuum, that HAS to be the culprit.
The overflow cap seal isnt that important but the radiator cap must seal for the system to function properly. Replace it and check the connections and hose to the overflow tank.
The overflow cap seal isnt that important but the radiator cap must seal for the system to function properly. Replace it and check the connections and hose to the overflow tank.
I already replaced the rad cap. I said that in my first post. I need to double check the overflow line.
Update. I vacuum tested the upper and lower hoses. They are both working correctly. The hose that goes to the overflow holds vacuum perfectly. I bled some air out of the other hose, I will drive it once this rain clears and report back... I have a feeling this wont change a thing.
Ive never dealt with this on an LT1, soooo i cant get too specific here.
If you have verified the plumbing from the overflow tank back to the rad cap isnt leaking anywhere, then you most likely have a leak somewhere else above the waterline. Example, on L98’s, when the intake manifold isnt sealed perfectly, usually at the rear near chl 7 or 8, there can be a small pocket of gas (steam) form, and it will equalize the pressure by sucking vacuum through a very minor perforation in the intake manifold seal there, rather than draw the fluid back in from the overflow. The long term prospect of this are an issue, as the gas pocket grows everytime there is a heat cycle. The overflow tank keeps getting fuller.
if there is a spot with a leak that is exposed to gas, it will preferentially flow before the fluid in the overflow does. because gas has zero viscosity.
now im not sure where those leakpoints are on an LT1. Not sure if LT1 intake manifold has coolant running through it.
On an old 87 that i had, i simply pulled the plenum and retorqued the manifold to head bolts and problem solved. the bolts were extremely loose.
so my point is, there will be a leak in the system. preferentially pulling gas in rather than the liquid from the overflow. 9 out of ten times, its the plumbing from the overflow or the cap that is compromised. good luck!
Ive never dealt with this on an LT1, soooo i cant get too specific here.
If you have verified the plumbing from the overflow tank back to the rad cap isnt leaking anywhere, then you most likely have a leak somewhere else above the waterline. Example, on L98’s, when the intake manifold isnt sealed perfectly, usually at the rear near chl 7 or 8, there can be a small pocket of gas (steam) form, and it will equalize the pressure by sucking vacuum through a very minor perforation in the intake manifold seal there, rather than draw the fluid back in from the overflow. The long term prospect of this are an issue, as the gas pocket grows everytime there is a heat cycle. The overflow tank keeps getting fuller.
if there is a spot with a leak that is exposed to gas, it will preferentially flow before the fluid in the overflow does. because gas has zero viscosity.
now im not sure where those leakpoints are on an LT1. Not sure if LT1 intake manifold has coolant running through it.
On an old 87 that i had, i simply pulled the plenum and retorqued the manifold to head bolts and problem solved. the bolts were extremely loose.
so my point is, there will be a leak in the system. preferentially pulling gas in rather than the liquid from the overflow. 9 out of ten times, its the plumbing from the overflow or the cap that is compromised. good luck!
This sounds most logical. I will check this stuff when I get home. Thanks for the input.
The same thing happened to me. The coolant overflow tank does not operate. Observe that the water did not move from one tank to another and what happened is that the tube was clogged by the years that my 92 vette has.
I unplugged it. Add cleaning vinegar and leave it for a few days and then with a push wire until you remove the blockage from the tube.
Last edited by Javi_Vette92; Dec 15, 2021 at 12:52 PM.
If the siphon is not working, there is a break in the closed system. That simple. You have tested for any leaks, but your test does not
include all components. Place news papers under the engine to see in any coolant is leaking from out of sight components (knock sensors in the
bottom of the engine).
A common leak spot is the rad cap gasket, but you have replaced that item.
Coolant system mystery?
