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1958 - 1981 Pontiac Tech & Restoration Firebird - GTO - Fullsize

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Old 06-06-06, 03:24PM   #1
g-man65
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Default what does this mean ?

curious as to why, after taking my car for a run,....as it cools back down to cold, the upper radiator hose sucks almost to flat,....never noticed on any other car before,....does it mean something??....the car does not overheat......at cold , theres water in the upper tank, almost to bottom of cap neck,...7# cap....sometimes it will seep a little out of cap while cooling down, not a big issue,...hardly any water is in the overflow tank.....just wondering??...g.
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Old 06-06-06, 05:17PM   #2
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Thermostat is stuck closed. I had the same thing happen to mine years ago. The water pump is basically pulling a vacuum through the radiator, because the water is not leaving the block back to the radiator.
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Old 06-07-06, 03:53AM   #3
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hmmmm....so its closing up early on cool down??....i had it out just a few months back to put a new water neck on & i tested it in a pan of water, opened right up as it should.....if it was staying closed all the time, i'd be over heating, right?.....its a quality brand high-flow 160 degree....g.
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Old 06-09-06, 02:00AM   #4
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Default Partial Vacuum

Get an upper hose that has the spiral anti collapse spring in it or one that is wire reinforced.
Drill a 1/16" hole in the separator plate of the thermostat.
Try a new pressure cap with a higher rating.

Too much coolant expanding out of the system and not enough air or additional liquid from the recovery resevoir can get back into the system as it cools. Might be a blockage on the recovery system there.Most closed system radiators have a "fill to"line on them.The tanks on the radiator need some open space or headroom in them for the hot system liquid to expand against.

As a closed cooling system cools it will produce a partial vacuum or negative pressure ,with respect to atmospheric , in the system , thats how the coolant recovery system works.
A partial vacuum has to be present in the system to create this condition and that is normal.

The same principle is used to induce a vacuum in home canning jars.

Steam heat exchangers will do this also when they cool down. Special vacuum breaker valves have to be fitted to them so they will operate correctly and to protect the steel shell or ????? from collapsing.

An electric water pump left to run after shutdown could also possibly cause this if the stat shut tight as it cooled down and the pump was dead heading. a 1/16"Relief / bypass hole in the stat plate along with the reinforced hoses would lessen the effect of that as well.
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Old 06-09-06, 04:24AM   #5
g-man65
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Thanx for the excellent information !!.....I was thinking about that hose, no wire in it at all....with my older radiator, do i dare push it with a higher pressure cap?...i added the recovery tank, don't know if its supposed to be a closed system or not...if i can't find a ''fill to'' mark, won't it purge any extra water out anyway?....i'll check the stat next time i have it out, but i think it has a small hole in it already...no electric pump......g.
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Old 06-13-06, 01:10AM   #6
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i have a customer with the same problem in a small block corvette.
it was also teh first time i'd heard of this before.
solved his issue with a spring, as noted above.
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Old 06-19-06, 06:35PM   #7
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Default colapsed upper radiator hose

Hey guys, I thought the old type radiator caps that were not on recovery type systems worked like a check valve. It allows pressure build up for higher boiling point but on cool down vacuum pulls open the check and keeps the hoses from collapsing. When you buy a kit to install a recovery bottle it has a new cap that allows the coolant to both expand and contract through the recovery bottle. The spring is in the lower hose to prevent it from collapsing when the water pump is sucking.
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