I recently made a 3,000 mile road trip in our 1996 3.9L five-speed without a problem on the highway (now has 299,000 miles and purchased new by my father). In recent weeks, I have added almost a gallon of coolant/water two different times in the radiator and overflow bottle. I borrowed a coolant system pressure tester from AutoZone and it seemed to hold pressure at 16 psi. I have never seen a leak anywhere either. Where is this liquid going? I have not noticed any decrease in engine performance or drive-ability either. This is my daily driver, so I need to fix this problem before it balloons into something major. Thank you.

 

There is a bypass hose under the a/c compressor, that likes to rot out, and leak, but, the coolant pools on top of the timing cover, and boils off, before you see much of a drip on the ground. Though, one would think, the pressure tester "Should" catch that one.... Another option is a failing head gasket..... You can get a test kit for that at your local parts store......