Get a cooling system pressure test and cylinder leakdown test done. It isn't impossible to crack a 4.0 block but in normal use it isn't easy.

 

so you still haven't diagnosed the problem properly?

pressure check it and see where it's leaking. that will tell you everything you need to know.

 

the head was a problem thats why i did it but its not the whole problem.

 

So you pressure tested system including the 10.00 radiator cap?

 

????

 

I've been having a similar problem lately and don't want to buy a pressure test kit for this. And, I'm praying that it's not the head gasket.

My wife first noticed it when one day as she parked she noticed coolant spilling out below the reservoir. I couldn't find any leaks though.

Then a week later it happened again and it was obvious that the coolant was boiling vigorously in the reservoir.

The weird part is that it only ever happens when the air conditioner is on. If we leave it off, it runs like a champ, even runs cool now that I changed the fluid and the thermostat.

But, if you turn the air on, it starts heating up steadily. When you turn the air off, it drops fairly rapidly.

There's still no coolant in the oil or oil in the coolant thankfully. Anyone have any idea what this could be?

Oh, we do have a small crack in the rubber of the radiator cap. We're getting all new hoses and hose sleeves this time, plus the new cap, on payday. I sure hope that takes care of it.

Added: I almost forgot the bubbles. When it's running there are small bubbles that rise up in the coolant res. weird

 

I had the same problem in my 99 XJ. Replaced the thermostat, well that didn't help. Than I replaced the fan clutch and that solved my problem.
It would overflow on a hot day after driving for a while and than parked the car, I'd come back a few minutes later and noticed antifreeze on the ground from the overflow. But I digress, the fan clutch solved my problem.

 

If the bubbles are consistent and always bubbling--not a good sign. A pretty good indicator of a compromised head gasket.

 

Well, it only does it in the reservoir. When I run it with the radiator cap off, there are no bubbles.

Seems to indicate the cap doesn't it? I sure hope so. Thanks guys.

 

Is the electric fan running when the a/c is on? How old is the fan clutch on the belt driven fan?

 

Wonder where the OP went?

 

Yup, I had bubbling in the reservoir one time and changing the rad cap fixed it. Basically the rad cap wasn't holding pressure in the cooling system any longer allowing water to boil at a lower temp.

 

It turned out to simply be a bad fan clutch in my case. All I had to do in the end was look on youtube how to check the fan clutch safely. I recommend anyone with this problem try this simple test before paying for special tests on your engine.

I wish I'd known about this months ago when I asked here! I could have had air all summer. doh

*Oh yeah, and it's only a $35 part. double doh