Overheated, diagnostics not showing anything

So I've got a 2000 Ranger with a 4.0 and when I went to work a freeze plug blew out and it overheated. Got it towed home expecting the worst so I put in a rubber plug, filled with water and it fired up and ran fine idling for about about 30 minutes. Had some bubbles in the reservoir that went away eventually so I figured maybe I got lucky.

Replaced the rubber plug with a real freeze plug and let the sealant dry 24 hours. Changed the oil and filter then put antifreeze in it and it wouldn't start. I pulled a spark plug and it was dripping gas so I pulled all of them, blew them off, reinstalled and it fired right up.
After it sat for awhile I couldn't get it to start again. I started thinking maybe coolant is getting into 1 or more cylinders and causing the starting problem so I removed all plugs and turned it over thinking it might shoot coolant out of the spark plugs holes, nothing. Replaced plugs and got it running and left the radiator cap off, it's puking coolant out of the radiator. Obvious sign of a head gasket/head issue. So I started testing to see if I could figure out if it was one head or both and I'm not getting any results that tell me much.

I tried pulling the wires on one side to see if it pressurized the cooling system, it did for both sides.
Did a compression test, 125 psi all around.
No white smoke.
No contamination in oil or coolant.
No evidence of coolant on any plugs.
I got a combustion leak detector but couldn't get a good result I don't think because I could only pump the bulb for maybe 30 seconds before coolant got pushed up into the tester, I did leave the level low in the radiator. But never saw the test liquid turn yellow.
Leak down test showed about 15% leak down all around and never pressurized the cooling system

I have found that the engine will start right up if the cooling system is empty, obviously I shut it right back off.
When I do get the engine running it runs great, I've test driven it and it seems fine. This is a work truck with 215,xxx miles so engine isn't worth tearing into to me.

Anybody got any ideas? I know the puking coolant system screams head gasket or cracked head but no tests are helping prove it to me so could there be any other causes for this mess?

Thanks for any ideas...

Do the Glove Test, it will confirm head gasket leak or cracked head and ID which cylinder(s) is leaking

Cold engine, spark plugs installed
Removed Rad cap
Remove overflow hose and block it's port on rad cap opening with vacuum cap or gum or putty or ??

Get a latex glove and rubber band or a balloon or even a condom
Place glove over rad cap opening and seal it with rubber band
Cooling system should now be sealed, doesn't need to have coolant in it

Disable spark, pull 4 wire connector on coil pack, you want a No Start

Crank engine and watch the glove
It will bounce if you have a head gasket leak or cracked head
Each time that cylinder is on compression stroke glove will bounce

If it does bounce then start removing 1 spark plug at a time and crank engine
When bounce stops last spark plug removed was from leaking cylinder, reinstall to confirm

If glove should just bounce less after a spark plug is removed then you may indeed have more than one cylinder leaking, not common but can happen.
make a note of that cylinder and continue test
Once glove stops bouncing you can replace those spark plugs and test them together and then separately

Thanks RonD, I had thought about doing that but figured it wasn't necessary with all the other tests and never thought about narrowing it down like that.
I'll grab the supplies when I go out this afternoon and try it once I get back.

Sometimes the old, and free, ways are best

Observation, of bubbles, is a good start, chemical testing can be OK but can also give confusing results, "what color is that?", lol.

Cooling system is suppose to be a closed system when rad cap or "glove" is in place.
So should remain at 0psi while engine is cranking.
Cooling system pressure comes from coolant heating up(engine running) and it starts to expand in volume, like water in a pressure cooker does.
Water pump is a water circulator, no "pumping" is involved.

So Glove Test is a pretty definitive test for a cylinder leak into cooling system.

Your No Start with fluid in the cooling system is puzzling though, even if one or two cylinders were leaking the other 4 or 5 should fire and engine should start.
If engine stops while cranking then that would be a hydro-lock symptom, so yes fluid is leaking into a cylinder
But your spark plug removal and cranking would have detected that

The glove confirmed it, started dancing for the number 3 cylinder. It's crazy that the compression test didn't show it.

Thanks again RonD

Good work

Thanks but all I had to do was sit here and type

Compression tests and even Block test(chemicals) don't always pick up smaller leaks.
They tend to work after you are billowing white smoke so don't really need a test to confirm it, lol.

4.0l OHV heads have a weak/thin casting between valve seats, if head is over heated and metal expands too much they tend to crank there.
So after you get the head off check #3 for a line between valve seats, no need to pay money to have head pressure tested if you see it.

1998-2000 4.0l OHV engines have different heads than earlier years, smaller exhaust ports, so have to be replaced with that model head

I was knew something was very wrong, it's just nice to have it confirmed.
I haven't decided if I want to even invest the time and money on an engine that got overheated like that with 215,xxx miles on it tho. It's been maintained pretty good but I'm eyeing a used one with a hundred thousand fewer miles for $500.
That or get another work truck and turn this one into a toy....

It's always a hard decision with these older vehicles. Spend the money, or scrap the thing?

My 98 is what I consider well maintained, even though on the outside it looks pretty bad. That's Oklahoma weather for you, hail dents. That's the main reason why I keep it, it's pre beat up. This way I can leave my 2004 Lightning in the garage, since it is a garage queen.

Last year I did fuel pump and filter, power steering pump and fluid, transmission fluid and filter, differential fluid, new back brakes, new back tires and spark plugs, new IAC, DPFE, IAT, CPS and CKPS. Including getting it diagnosed for a crank-no-start condition, I spent over two grand. No way I can sell a 98 Ranger 3.0 V-6 with 147,000 for that kind of money. So it's like, once I decided to do those things, it becomes a keeper until something big, such as a warped cylinder head enters the picture. At that point, I would probably pay someone to tow it to a scrap yard.

Forgot, in February, I re-did all the anti-sway bar frame bushings and end links. The old ones were in really poor condition, and the bushings on the front bar end links were either missing or fell apart in my hands when I took them off. The rear bar is a Hellwig I put on in 2001, and even those parts were in piss poor condition. That costed another $100 or so, but was well worth it, handling is improved big time.

Is there any reason a 98 Explorer engine wouldn't swap into this Ranger?

1998 Explorer could have a 4.0l OHV or a 4.0l SOHC....or a V8

4.0l OHV would be a straight swap, 4.0l SOHC would not

You would probably have to swap oil pans and intakes, just have to eyeball if they are different