coolant overpressurization, replaced parts, glove test

[vagabond80]

Hello

I'm in a bit of a pickle.
The heads on my '90 2.9 were magnaflux tested and had the gaskets replaced this past spring. Shortly after, I replaced the radiator with the 4.0 automatic a/c giant out from the explorer, and a new 13lb motorad cap. Never gets hot, in fact it would typically run up to the middle of "normal" on the gauge (after quite some time), then bounce back down to the "c" area.
Although it may have been happening for some time, I eventually noticed a small coolant leak which I traced to the lower rad hose. I tightened it and things seemed fine for a drive or two, then I noticed more coolant. First from the upper rad hose, tightened, then saw it leaking again. This time from the thermostat housing. I reseated it, drove, still leaking. Resealed it again, this time using permatex aviation form-a-gasket on the machined surfaces. Seemed good. A couple drives later(its pretty cold where I am) the radiator blew after shutting down the truck.
A couple days ago, I replaced it with a known-good "normal size" radiator from my other ranger with <1000 miles on it, and a new 13lb motorad cap. Filled and ran with the cap off, til the thermostat opened then filled more but ran out of coolant a few inches shy of the top. After that, drove from my work to my house about 20 minutes, no leaks. Didn't get hot on the gauge.
The next morning I drove to my storage/shop and filled the rest of the radiator, and because I was paranoid, I swapped the new 13lb cap to the one that was on the "known good" rad, an 18lb cap(every aftermarket I've seen is 13lbs, the motorcraft is 16lbs, I don't know where this one came from but it "fit" and worked on a 700 mile drive in the other ranger [til the trans broke]). Following that I drove a few miles back to the house, stopping a couple times at some stores. No leaks. Maybe half a mile from my house, I saw the gauge climb just to around the "a" of normal and then swing back to around the "n" where it had been. When I parked in front of my house, the radiator began to just dump. Not from hoses or cap.
There are zero replacement radiators within an hours drive.
I just did a "glove test" that appeared to check good, but I'm concerned my radiator may be too blown open to get good results.

I've read now that the heater line needs to be opened on at least the 3.0 to purge air. What are the chances a new radiator (Tuesday at the earliest) will not blow if I purge the air "correctly"?
I'm running out of options
Thanks for any input

 

[Paulos]

Head gasket, thermostat stuck in the closed position, non-diluted coolant? I'd first check the thermostat to make sure it opens and closes correctly (in a pan of water on the stove, using a thermometer). Any bubbles at the radiator with the cap removed and the engine running? If it's the head gasket you may have to get the engine to operating temps before it will start blowing air into the cooling system and out at the cap. An 18lb cap sounds a bit much. I've always used a 13lb without issues. I also have the 4.0 auto/air radiator without issues.
That Ford "Normal" temp gauge is practically useless. You need a real temp gauge so you can tell how high the temp gets before the t-stat opens. There's a lot of debate on whether you should run the stock 195° or a 180°. I've always used the 180°.
If the t-stat is good, I would have to guess it's a head gasket or cracked head issue.