bubbling coolant in overflow

[Lumpy Grits]

I'd be look'n at do'n a leak-down test, PDQ.
What's the engine oil look like?
LG

[daniel_buck]

My TJ bubbles in the overflow tank after driving 20+ miles when I turn the motor off, has done this for a while now, the temperature has always been good, right smack on 210 (which I gather is perfectly normal for TJs) and the level in the overflow tank hasn't ever been low, so I didn't pay much attention to it.


Recently, it's started bubbling after just 4-5 miles. So, I figured it's probably the radiator cap that isn't holding pressure. Put on a new cap, and I noticed that even though the level in the overflow was "full", I didn't see any coolant in the radiator. I didn't have any coolant or distilled water with me, so I just drove home with the new cap. And no bubbling when I got home. The new cap has a release valve on it, so out of curiosity, I pulled the release valve as soon as I got home, and the same bubbling sound happened that used to happen with the old cap. So... I'm fairly certain that it was just the old cap that was not holding pressure.

However, since I didn't see any water in the radiator itself, I let the motor cool down, and then this morning I filled up the radiator until I could see water near the top. With all of that out of the way:



So my question is ---------> why would the overflow reservoir read "full" (after it sat overnight to cool down, which I assume is when the fluid would get sucked back into the radiator?), while the radiator itself wasn't full? I was able to put about 1.5 quarts into the radiator itself. Which I believe means the system was only about 90% full. Which might explain why the bubbling has recently started sooner than it used to with the old cap?


Preemptively, I'm going to order a thermostat and a waterpump to keep with me on long trips (I do drive across country once every year). My jeep has 140k miles on it (2006 Rubi 4.0L). As far as I know, the thermostat and pump have never been changed, and the radiator fluid has only been replaced once. Would it be a bad idea to just go ahead and preemptively replace the thermostat and waterpump and flush the cooling system now? or just wait until heating problems actually occur later on down the line?