Y-Block thermostat

Daves55Sedan

Daves55Sedan

Well-known member
I did a cooling system flush and refill but also tested the thermostat in heated water. It worked okay so I put it back in. after running engine for awhile I noticed the top radiator hose is under high pressure and looks like it is going to pop. You can't even squeeze it, like it is solid the pressure is so high, even after shutting off the engine it stays like that for a long time.

Does the thermostat need to be rotated a certain way in the elbow? If so, I never paid attention. I do have it installed so that the spring is toward the engine and the valve is toward the radiator.
 
Well, I just ran back out to the garage and looked in a box labelled "cooling parts" and there are two new thermostats in there. one is a 160 degree and the other is a 180 degree. Niether one of them have any markings indicating "TOP" or BOTTOM", but the 160 has a marking indicating which side to aim toward engine. And I have mine installed correctly like that. So I am stumped. Could the radiator be clogged up that bad?
 
I Hope the bypass is not clogged. That's a 5/8" hole thru there. If it is clogged, then surely the radiator must be.
 
Couple questions. Did the engine come to operating temp, or did it overheat? If the bypass is blocked, I'm not sure what the effect would be, but turning the heater on high provides the same water route as the bypass. Have you tried starting cold, with the radiator cap off and looked for water flow thru the top tank?

Someone either here or on another site suggested removing the radiator cap (cold), then pulling the lower radiator hose. If the radiator doesn't dump rapidly into the a bucket under the hose connection, it's a plugged radiator.
 
Just my 2c, but wouldnt a plugged radiator give some warning as the water flowed less and less, like growing overheat symptoms?
Mikers suggestion is good and easy/fast to checkout.
Hope you clear it.
 
Well, the bottom hose is the suction end and the top hose pushes coolant from the engine into the radiator via the bypass tube prior to the thermostat opening. After the t-stat opens, the pump pushes water into the intake manifold and down thru the engine block.
So, the problem can only be two things:
Either the radiator is completely plugged up or very badly at least, or there is a blown head gasket releasing exhaust gas into the cooling system.
When filling the radiator, the water does make it down to the bottom, so it is not completely plugged, but if several lines in the radiator are plugged it might be enough to cause the high pressure.
I think the thermostat can be ruled out as the problem, in fact it seems the pressure increases greatly after the t-stat opens.
What test can I use to determine which is the problem.
 
Borrow another unsuspected radiator. Fill yours and the "new" one. And check:
a) how much water does each one take
b) how long does it take for each one to drain it all via the bottom tube.
Id try this simple test. It might give you some lead.
If very different results appear, try the borrowed one and see what happens.
 
Daves55Sedan said:
So, the problem can only be two things:
Either the radiator is completely plugged up or very badly at least, or there is a blown head gasket releasing exhaust gas into the cooling system.
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Take the rad cap off when the engine is not hot or running, then start the engine and let it run with the cap off. You should be able to see the coolant moving across the top of the radiator when the engine gets to operating temp. With any luck you'll also be able to see the open top ends of the vertical tubes inside, they shouldn't have any crud buildup on or in them.

If a blown head gasket is pressurizing the cooling system you'll get lots of bubbles and coolant blowing out of the open cap. (and coolant into the cylinders)

 

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Here's a video. He's working 1 handed and drops it twice (haha) but he puts it in the right way and the 302 is nice looking.
https://www.youtube.com/watch?v=kNrBud_Fh-8

If there's so much pressure in the cooling system that the upper hose feels like it will pop you probably also have a bad radiator cap or plugged overflow tube. Depending on the cap's rating anything over 5 to 12psi should go out the overflow tube.
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A "pressurized" cooling system is the result of - building pressure with little or no vent. So you need to test the cap - not the T-stat. The earlier photo posted on this thread shows a radiator tank - mostly empty - with lots of room for expansion. And that is the way these systems were designed - the "overflow" we see on later models was actually accommodated with the extra space in the tank. If you fill one right up - cold - and then the cap doesn't vent correctly - it will definitely pressurize the upper hose.
 
Well, it has to be a blown head gasket, because the only way to depressurize the top hose is to run with radiator cap off, but it blows my coolant out if I do that. If I leave the cap on it completely fills up my overflow bottle and blows the cap off of it. Had me stumped though, cause normally with a blown head gasket, she'll blow a huge amount of steam out of the tailpipe when you let off the gas on the highway and it is not doing that. Which must mean that the head gasket breach is just between and exhaust valve and a water hole (not the cylinder itself).
 
Daves55Sedan said:
Well, it has to be a blown head gasket, because the only way to depressurize the top hose is to run with radiator cap off, but it blows my coolant out if I do that. If I leave the cap on it completely fills up my overflow bottle and blows the cap off of it. Had me stumped though, cause normally with a blown head gasket, she'll blow a huge amount of steam out of the tailpipe when you let off the gas on the highway and it is not doing that. Which must mean that the head gasket breach is just between and exhaust valve and a water hole (not the cylinder itself).
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Just wondering out loud over here.... If, as you suspect, the pressure leak is between the exhaust and a water passage.... might that be in the head itself and not the head gasket?
 
Thermostat is not the problem with pressure in the upper hoses. The pressure cap
should release excessive pressure over the cap opening pressure. You should not have over a pound or two of pressure in a short time of running if the temperature is below 205 degrees. Your problem sounds like a compression leak. Remove the cap, fill the radiator so the water is up in the filler neck and start the engine, watch down in the water for bubbles they indicate a compression leak. You can not see them if you don't fill the water up into the filler neck. Also have your pressure cap checked you don't need over a 4 lb cap. The cap is sealing at the bottom, if it wasn't pressure wouldn't build up so much and the water would go out the over flow tube. G.M.
 
Daves55Sedan said:
Either way, compression leak or exhaust valve leak to a water hole in the head, the fix is a new set of head gaskets, right?
Click to expand...
While you have the heads off it wouldn't hurt to have them double checked for cracks into the water passages, especially around or even behind the side-by-side exhaust valves at the center of the head.
Look halfway down the page at this link...

http://ford-y-block.com/troubleareas.htm

Possible helpful fix info in the 4th & 5th paragraphs on this page, tho I've never heard anyone mention it before.
http://www.enginebuildermag.com/2002/05/working-it-all-out-discovering-y-block-ford-tricks/

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