Hey my 99 WJ 4.0 has been loosing coolant for about 25,000 miles.....

Should I replace the head or just swap the motor...its been running fine, still runs fine. The oil sensor did die on me but that was no big deal, I thought it was just the water pump, but under the oil cap is milky so figuring it's a cracked head or head gasket. Just not sure what route I should take.

Assuming you have absolutely no small external coolant leaks that would account the coolant loss and that you have milky crud on the oil cap there is a good chance you have done damage to the engine. It does not matter if it was a bad head gasket or cracked head. If the problem has been present for 25,000 miles that is likely a lot of extra wear due to coolant contamination of the oil. I would suggest getting a oil sample analyzed. That will show if coolant is present in any significant amount. It the results come back positive for coolant in the oil and if wear metals such as iron and led are high the bearings and cam shaft have almost certainly taken a beating. In that case rebuilding that engine or a different engine with known history would be in your future. I have been very satisfied with Blackstone Labs for oil analysis. Best $28 you will spend in determining the best way forward.

Try the old 'radiator pressure tester trick.' its free.

Thank you! I ordered a test kit and will go from there I suppose. I think this makes the most sense, I agree. No point chasing the problem (or the resolution) until I confirm the damage.

Will Blackstone labs tell me if there is to much of lead or iron? Do you know what I should look for?

I was going to untili saw the milky under the oil cap. I'll post a picture. Hang on.

Look like coolant to you?

You know what, it over heated realllllyyy bad (the last and third time) damn remote start....I was in the basement picking my kid up and used remote start to keep it warm. I came out to practically a fire.

I bet whatever was causing the coolant to leak in the year and a half priorto that instance was small, and since then it's been really bad. So maybe it hasn't been mixing oil and coolant for 25,000 miles, maybe it's been leaking coolant for 25,00 miles, and mixing for about 4000-6000 miles.

Yes, Blackstone will tell you what the part per million is of the wear materials and if this is ok or way out of normal range. They will also identify if there is antifreeze in the oil. They provide a great report.

Excellent thank you!

Been there, done it! Just wanted to let you know I agree with everything that's been said in this thread. I got lucky with mine. It only cracked into an exhaust port! Lots of steam and coolant dripping from the tailpipe.

Replacing the engine with another used unit can get you back on the road very quickly for the lowest cost. It also is the riskiest solution since you can't know all the history of the replacement engine. Similarly, replacing your engine with a rebuilt unit can also be done quickly, however at much greater cost. Rebuilding the original and replacing it will be the most time consuming as well as expensive.

Digging into the actual source of the failure and addressing it may be the least expensive option as you are only replacing the damaged parts, but is also highly dependent upon your ability to isolate the issue and replace only that item. It also carried the risk of early failure if there was any periphery damage that is somewhat undetected. The oil analysis will be a big step in understanding that risk.