I recently replaced the coolant sender (for the guage cluster), as the guage never went past the center. I knew this was not correct, as I had installed a e-fan recently, and as the engine idled to test the 2 speed fan (I was testing the temp probe sensitivity adjustment), I knew the coolant got hot, as I began to see some coolant misting out from a fitting on the drivers side of the radiator, used for one of the automatic transmissions oil cooling lines.

So, I loosened this transmission line fitting, then was able to tighten the fitting to the radiator almost two turns. I suspect that when the Ford dealer I bought the truck from (used), changed the water pump, they removed the radiator to clean it (does not look to be a new replacement), and did not tighten this fitting down upon re-install.

After replacing the coolant sender, I removed the radiator cap and saw that the coolant level in the radiator was low, and suspect it was leaking from this fitting; even though I never saw fluid on the ground after a drive, nor any coolant on the engine near this fitting, I suspect this was where it was loosing coolant.

My question is this: why didn't the radiator "draw" fluid from the overflow (or as Ford calls it the 'degause') tank/container? The coolant in this container is between the cold and hot fill marks. I would think because of the amount the radiator took with me filling it to the lower part of the neck, it would have drawn from the overflow tank. The tube from the radiator neck to this overflow container is there. Could the radiator cap be incorrect (too much of a pressure rating)? What should the pressure rating be for these caps?

 

The overflow system works like this:

The rad cap has two valves in it, the larger pressure valve, usually 14-16psi out, and the smaller "center valve", always 0psi IN.

As coolant heats up it expands in volume, this is what creates the pressure in the cooling system, if there is a 14psi rad cap in place then at 15psi the cap is forced open and hot coolant is sent to overflow tank, at 14psi coolant stops going to overflow tank.

After engine is shut down the coolant cools off and shrinks in volume, so pressure in the system drops, 14psi, 10psi, 5 psi, 0psi, -1psi, at this point regular outside air pressure(15psi at sea level) pushes overflow tank coolant back into the rad via the "center valve" in the rad cap.
So coolant is "sucked" back into the rad as cooling system pressure becomes less than outside air pressure.

This is a self purging system and works quite well as long as system is "air tight" and overflow hose is always covered by coolant in the overflow tank.
Any air in the rad will be at the top, as coolant heats up and expands above 14psi the air will be the first to be pushed out, it will bubble up in the overflow, and when engine cools down, coolant will be sucked back in, so air will be purged.

Air is easier to move than a fluid, it weights less.
If you have a leak in the cooling system then when under pressure coolant will come out of this leak, and when cooling down AIR will be "sucked" in to the cooling system thru this same leak, because the air is easier to "suck" in than the coolant in the over flow tank.
This is why you had low coolant level in the rad when cold.

On my '94 I had a small crack in the overflow hose, it runs across the top of the rad.
When engine warmed up, above 14psi, the hot coolant would drip out of crack and evaporate on the hot rad, so no sign of a leak.
Then when engine cooled down it would "suck" air in, instead of coolant, so rad would show low level after cooling off????????

Was a b*tch to track that one down, lol
Rented a pressure tester and could let it sit for hours and it wouldn't lose a pound of pressure, system was tight but still had low coolant level after cooling off................
Even pulled off overflow hose and blew in it, bubbled up nicely in overflow tank, but never tested it by removing both ends to see if it held pressure.
So I just keep topping off the rad every few days in the morning.
I was doing some other engine work and removed overflow hose completely off the rad and keep end just above overflow tank level so coolant would drain out.
When reinstalling overflow hose I noticed hose being wet in a place that shouldn't be wet.....................EUREKA, wrapped some tape around the crack and it has been fine for a few years now.

 

Amazing knowledge base you have! This makes perfect sense now that I read it and understand the fact that any leak would draw in air verses coolant from the overflow tank. That upper fitting on the drivers side of the radiator for the transmission oil line that was loose was likey sucking in air as the radiator cooled, thus explaining why my radiator was low (that loose fitting was also loosing the coolant; a double edged sword). I think this weekend I'll remove that fitting, put some teflon tape on it, reinstall, then run the truck for a couple of days and check the radiator to see that it stays full. I should replace that long piece of rubber tubing from the radiator filler neck as well, since it appears to be the original one, just for safe keeping.

Thanks for the education; feels good to learn something. Even an old guy like me can have a good day learning!

 

Thanks for the kudos

And just to fill in some gaps.

If upper rad hose collapses as engine cools off then "center valve" in rad cap is bad or overflow tank/hose is plugged up, this can happen if you get debris in the tank, coolant can come in, it pushes debris out of the way, but it can't get back out to rad because debris settles at the bottom and plugs the hole, you end up with negative pressure in cooling system(below outside air pressure) and 15psi outside air pressure collapses hose.


Pressure in the cooling system is to raise the boiling point of the fluid.
Water boils at 212degF
50/50 coolant/water boils at 220degF

Each 1 psi of pressure raises boiling point by appox. 3degF
So a 15psi cap adds 45degF to boiling point of any fluid, for 50/50 that would make boiling point 265degF(220 + 45)

Outside air pressure is 15psi at sea level, at 4,000ft it is 13psi, so you LOSE 6degF in the boiling point if driving at 4,000ft, keep that in mind.

Engine design can have issues, "hot spots" can develop near the cylinders domes in the heads, manufacturers usually have a rad cap pressure recommendation, you should use this even if it seems high, if engine does have a hot spot coolant can "flash" to steam if pressure is too low, if that happens usually a hose will blow off, or overflow tank will...........overflow, lol, as higher steam volume forces out the coolant.


Some vehicles use an "expansion tank" instead of an overflow tank, the difference is the expansion tank has a "rad cap" and is not open to the air.
The expansion tank has some air in it that compresses as coolant volume increases from heat, rad cap on these just has the one valve, for rated pressure of system.