Did magic push it out.....not. The heat soaking of the motor raised the pressure and temp of the coolant making it expel out the overflow. The cap either was not sealing properly or not high enough pressure. Mystery solved!
My 327 I might mention had no sign of overheating as Paul's car. I could remove cap and let it idle probably all day and would never even raise the coolant level . In my case it was the cap ,period. In Paul's car I bet you could not remove cap and let it idle without raising level and puking. I have done many water pumps ,change all cooling hoses flush cooling systems ect, in my wrenching days and I could always once coolant was refilled and idled till thermostat opened then removed cap immediately and top off coolant again . Once completely filled I could let it idle with cap off with no sign of level coming up. Now when head gaskets were bad you could not leave idle with cap off and it would push out coolant ,not very hot on a cold start but it was forced out eventually. Also discovered heater in car would not heat properly when head gaskets leaked and pressured cooling system. So if everything is right you should be able to remove a cap when thermostat has opened and cooling is circulating and have no antifreeze pushing out.
What head gaskets are using on that engine,Paul? Head gasket can leak pressure out but not leak coolant into the engine.Look at it this way,the most coolant pressure that system will see is less than 20psi,but the cranking compression is at least 160 psi.,and then the pressure that the cylinder sees when that cylinder fires.A trick that I've used for cylinder leakage was to remove drive belts,remove thermostat,take the top of an oil filter box and make a partition,insert the partition in the center of the thermostat opening,fill the manifold,and start the engine.Bubbles would indicate a leak and one could
easily see which side had the problem.Bubbles in the coolant = somethings leaking internally.
Don my head gaskets are Felpro full 409 set. I used Felpro sets on my last 3-409 motors (including my friends 409 motor).
They was not evidence of leaking on the face of the 690 heads(after disassembly) when I reviewed the head conditions with my machinist after 150 miles.
My radiator in the 62 SS 409 convertible is a brand new 4 core brass core 409 radiator using factory 409 hex top and 409 bottom bracket.
This new radiator was removed sent to the radiator shop for cleaning (even though it had only 150 miles on it). they then drove the car again and it pushed out coolant out of the overflow like its did every time I drove it.
I have another 409 4 core radiator in my 62 327/300 HP Impala that runs at 185 degrees all day in any driving conditions.
this 409 radiator has a 15 pound cap and never ever has overheated.
My convertible 409 brand new 4 radiator noted above has a 13 pound cap -the one that is overheating after it stops driving.
My other 409 SS Hardtop with the 500 HP stroker motor has a 13 pound radiator cap without any overheating problems. Just drove it on a loop night in town under 40 miles per hour in 90 degree humid weather for over a hour on a loop night -never overheated.
I intentionally installed 13 pound radiator caps on both 409's from 15 pound or higher before the 409 convertible motor was finished late last year.
My 409 4 core radiator that is overheating was never only 1/2 full of coolant, it was just down to the top of the brass core and also full to normal location. Either way it blows out the coolant when the car is stopped driving for 15 minutes or even a hour at operating temperature off 185/190 degrees on my Stewart Warner temp gauge. normally when it is driven without stopping except some stopping it will not overheat -goes up to 205 then back down to 190 when moving . Stop and shut it off it overflows out the overflow rubber tube.
I have another set of 690 heads even though this set with all new parts was crack checked(twice) and found OK
Pull the water pump off and then the cover on the back and make sure the impeller is not loose on the shaft. Had an Olds engine like that one time and it would overheat at idle then puke everywhere. Just a suggestion.
What was the cause of the score in the cylinder?If it was due to insufficient ring end gap this could cause a lot of extra heat rejected to the cooling system due to the extra friction.Another thing to look at would be the water pump if flow is too low at idle and then picks up as engine speed increases it would cause the symptoms you describe,even if the pump is new or rebuilt if it was assembled with excessive clearance between the impeller and the housing it could cause low flow.
As 425/409er stated check the impeller.
Another thing to check is the radiator opening to make sure it is made correctly. Maybe it is not letting the cap to fully seal and when you shut off the vehicle it builds pressure then pukes fluid???? Systems will heat up and build more pressure when shut down.
I would think if it was a head gasket or crack it would show up when running not shut off..
How are you checking temp? IR gun, gauge, where is the sender, head, intake, better gauge than factory???
The radiator is an original 1962 GM 409 radiator with a new brass core(it has mint factory connections). its not the issue.
As I said the water temperature gauge is a Stewart Warner full sweep gauge. I have them in all my 62's very accurate gauges -I also have oil pressure and fuel gauge all from Stewart Warner.
Water pump is fine, ran motor without water pump the engine will pump water out of the intake upper hose/thermostat outlet "without a water pump hooked up".
The heater is not bad, in fact the heater core is new and the heater valve is rebuilt. I just do not need a heater -so I leave it unhooked, as I do in my other 62's. Very little driving in cold weather.
We are going to reassemble the motor out of the car, with all new gaskets, no push rods, and no exhaust manifolds and pressure check each cylinder without starting the engine with water in the engine. All to isolate the problem after crack checking block and heads (after disassembly) and finding no cracks.
I depends how much is coming out just heating the water makes it expand greatly thats what causes the pressure buildup in the closed system.If it is just slowly dribbling out its probably just expansion,if its more than that it would indicate cylinder pressure leaking into the coolant system.