1970 Cadillac Eldorado coolant overflow again

It's happening again. A new cap helped for a while but since the weather got warmer, I get coolant overflow out the overflow tank once I turn the car off.
This is a 1970 Cadillac Eldorado (yes, the monster 500 CI motor with 400 HP). As long as the car's on, it's fine. Turn it off and there's monstrous gurgling, fluid surges to the overflow tank and not long after, right out the overflow hose. Strangely, I check the car once it cools down (circa 3 hours later) and the coolant is all the way to the top of the radiator cap. That'd ordinarily mean it's overfilled but I don't think it can be. Today I tried just turning the car on for 30 minutes (without driving it) and this happened. Suggestions? The radiator is maybe 5 years old, with very few miles driven on the car since then. One coolant change (power flush) since then, perhaps 2-3 years ago.
Thanks in advance to anyone who can help.
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The Derfer wrote:

How long since a tune-up by someone who knew what they were doing?
The first thing I would check is the vacuum advance in the distributor. If the advance diaphragm is still good (unlikely if it is original) then check the thermostat. Also check the lower radiator.
-jim
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are you sure you don't have any air in the system?
You can always try a new thermostat, or maybe put a lower temp one in.
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On Wed, 5 May 2010 15:51:40 -0700 (PDT), The Derfer

Sounds like you don't have a high enough concentration of antifreeze. Do you have an infrared temp gun you can aim at the thermostat housing to see what temp the coolant is at? The last vehicle I had that did what yours is doing was cured with a new radiator cap but you say you've already put one on it. Are you sure you put on the right pressure rating cap, it probably is supposed to be a 15 pound cap, maybe you put on a lower pressure cap then that. See if there's a gas station that has the gizmo that can check your coolant concentration to see if the ratio of water to antifreeze is at least 50/50.
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I seem to remember this topic from a long time ago.. I think we talked about hot spots in the engine, especially when cooling was marginal anyhow. When the engine was shut off, there was still enough heat in these areas to cause local boiling.
Now, what to do about it?
Troubleshoot systematically... Make sure you dont miss anything small... Correct (double check) thermostat? Water pump impeller? Lower hose collapsing leading to overheating? Mineral scale (includes silicates) in the block or radiator? Debris between the air conditioning condenser and the radiator? Air bubble in the engine? Cracked head, block, or blown head gasket?
I will try to search this symptom a little, because I remember it well, but dont necessarily remember the normal cause of it.
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The Derfer wrote:

xxxx I just now saw this.......I have a warmed up 500 in a 78 Seville.
When you shut down after a good run the stored heat will push coolant into the reservoir tank. If the reservoir is filled beyond the cold fill line it might push some coolant out of the reserve tank overflow which is normal. As the system cools the coolant will flow back into the radiator as the cap has two seals and this allows the system to purge air so if you remove the cap on a cold engine you will see the radiator is filled to the cap. That's when you check the reservoir tank to see it's coolant is at the cold line.
To my estimation this is the finest engine Cadillac ever built. 5" piston centers mean lots of cooling capacity as it isn't siamese as some engines are to save money. The 500 is about 50 pounds lighter than a 454 yet the block dwarfs the 454 Chev. The 4.3" stroke means lots of low end torque. My set up with a 200-4r and 3:08 cogs gives me 80 mph at 2000 rpm and it will gain speed up any mountain pass locked up in 4th. 17 mpg
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