1974 Eldorado coolant flow problem

I'm completely stumped here. I have a completely stock 1974 eldorado. the car has a brand new radiator, and new thermostat I just installed (yes facing the right direction). Randomly, something gets blocked up in the system and coolant/steam will shoot out of the upper radiator tube where it meets the thermostat housing. When it gets like this, the upper hose because rock hard from pressure and it will stay like that for several minutes before all steam is evacuated. Now, the interesting thing is, nothing is flowing into the coolant overflow can... that is.. until I flip the pressure relief valve on top of the radiator cap, then coolant and steam shoot into the overflow canister! I grabbed every coolant tube everywhere and they all seem warm, and they have to be because I know it is flowing at least some of the time because the heat/defrost works in the car.

this problem was happening before I replaced the old thermostat which I propped open thinking it was the problem, and it seemed to fix the problem, but when I replaced it with the new one I noticed it wasnt propped open anymore so this makes me think the problem is random and I cannot solely blame the thermostat.

Im thinking either I keep getting bad thermostats, or the coolant is getting blocked somewhere by something maybe in the head or heater core??? And why does my coolant overflow only work when I flip the valve on the radiator cap? In the tube that goes from the head to the heater core, there is vacuum operated valve with a vacuum tube heading towards the heater, I dont know what this is but could it be the problem??? Im stumped


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Might try a new, lower pressure radiator cap. Something like 15-16lbs without the pop off lever.

i HAD A 1970 ELDO with the 500cid and i replaced the water pump cause when i took it off it was plugged with rust "AHHHH" so i would replace water pump and drain the block of the coolants and try flushing out your block take thermostat off and spray a high powered hose "thumb over end haha" and get what ever gunk out u can... also not well known but it works one time i have had simple green in a coolants bottle and put in radiator ran the car and flushed never seen it so clean haha and it is eco friendly so in the drain it goes

Why would the water pump be at fault if the upper radiator hose is completely pressurized and hard to the touch? Wouldnt there have to be water pressure created by the pump to pressurize the hose in the first place? or is pressure from steam only... that I do not know yet. I guess a water pump is only $30 so maybe I should just replace it anyways. I like the idea about the radiator cap to SQZBOX, and that would probably help relieve pressure but I dont see how it will fix the random blockage, unless Im just not thinking outside the box

just for a test i would remove the thermostat
i would also check that "new radiator" something could floating around in it

Are you using the correct Cadillac thermostat?

The reason I ask is that the big inch Cadillacs (425/472/500s) take a special thermostat. Raher than running an external bypass hose that allows water to recirculate in the block for quick warmup, the bypass passage is located in the block under the thermostat. The correct thermostat has a disc at the bottom that closes off that port once the engine is warmed up. Not having the right thermostat will cause heating problems.

"..... upper radiator tube where it meets the thermostat housing......"

I'm tyring to picture what you are talking abo0ut here. Could you post a picture or a better description?

The valve you're talking about is simply the water control valve that controlls the amount of hot water going to the heater core and should have no effect on what you're describing.

my computer monitor crashed a few days ago, and I just got a replacement. I will take a better picture tonight or tomorrow and respond in detail

If the top hose is swelling up it has too much pressure and the cap needs to be one of the two way recovery type. Hot coolant expands and needs to go somewhere (the coolant recovery bottle). The two way cap will allow coolant to return to the radiator after the engine cools off. I would try a new cap first as it's cheaper.

So far, my thermostat was not the right kind, and the modulator for the egr valve was missing (i didnt notice because I've never had to work with one) and the vacuum lines were not hooked up right, and also the electric choke pull off was bad.. all problems that were discovered in the process. The right thermostat was that fit the block off hole perfectly was difficult to find.

On a different note I now I need the tranny shifter linkage bushings and I dont know where to find them, any ideas?