Hi,
Refill the radiator and then pressure test the cooling system. You will need a pressure tester, but most parts stores will lend one to you.
Here is a link that explains how it's done:
https://www.2carpros.com/articles/radiator-pressure-test
I have a feeling it could be a head gasket, so make sure the coolant isn't mixing with the engine oil also.
There were two years, and for the life of me I can't remember when, but the 4.0L were having issues with the cylinder head cracking between cylinders 3 and 4. I think it was 1999 and 2000, but I can be certain and I don't know what year the new engine came from.
I attached the directions below right from the manual for testing for a head gasket leak. Take a look through them and let me know your thoughts. Also, pic 1 simply shows where a gasket can leak. It's for reference only.
Take care,
Joe
See pics below.
Images (Click to make bigger)
Saturday, August 20th, 2022 AT 9:04 PM