Bear with me this might be a little long. I am a seasoned mechanic but this is my first Jeep. I bought a 92 XJ not running. It’s 4.0, AX15, 4 X 4. Got it running but it had rod bearing noise on startup and almost no pressure hot. I decided to build an engine for it. I had a 01 block and a 7120 head of unknown history. I had them vatted and crack checked. I built the engine around these parts. This build is with all stock quality aftermarket parts like Sealed power ETC.
So it starts and sounds good. I am breaking in the cam and topping off the coolant. All of a sudden a geyser erupts out of the open radiator filler. Coolant everywhere.
Cooling wise I have a new 3 core aluminum radiator, a new water pump with an “R” on the impeller, All new hoses. I am afraid they missed a crack.
Anybody have any ideas from their experience?

what happened after the geyser? it's possible there could've just been trapped air.

I immediately shut it off. I had not thought about an air pocket. The thermostat is new. I wonder if I should take it out and see if it makes any different

how long did it run before you had issues? I'd make sure the radiator fill is the highest place in the system via ramps, jack, or simply a hill and then start it up and continue doing what you're doing. do you have something like 2500 rpm for 30 mins on the cAm break in?

I measured the amount of coolant I put in (12 qt). So I don't think an air pocket was the problem, I ran it at 2000 RPM for about 15 min before it spewed out. It was just level not elevated.

I would suspect an air pocket too. I jacked the front of my jeep up a little and massaged the radiator hose until I saw no more bubbles.

typically a crack or bad assembly will be evident very quickly. if you're terribly worried about it you need to perform a leak down test. it will tell you exactly what, if anything, is wrong.

I'd fill it with coolant, elevate the front of the jeep and get back to break in.

I do not have a leak down tester but I put 30 PSI of air in each cylinder when they were at TDC. I listened for any noise. I did not hear any. So in the morning I will elevate the front end and try again.
Thanks for the help.

Air pocket.

I filled it up with water tis morning, elevated the front on ramps. took about 7 minutes for the geyser to erupt. I don't think an air pocket is the problem. I also think it gets hot too quick. I filled it with well water which is probably 65 degrees.
Any other ideas before I star pulling?

Need to put a pressure tester on radiator and pump it up and do a breakdown test. Parts store will rent you one. When you return it you get your money back. I bet your head you had checked is bad. A crack in head will put engine compression into the coolant passages in the head and will geyser out like you are describing.

Pressure test the radiator...

is coolant moving in the hoses?

I am pretty sure you are right about the cracked head. I think I will just quit fooling around and pull it. I have a 7120 off of a 92 that was running when pulled. I will take it in and have it reworked. I just hate the double work and expense.
Thanks

a stuck thermostat will not cause a geyser effect. If t-stat is stuck you radiator would stay cooler and your lower radiator hose would be hot and you upper hose cold..

If you rev it and geyser gets bigger then yes also if it kinda pulses out of radiator then its the head. But a pressure tester on radiator would tell you definitively