coolant leak while sitting
 

I have I had a slow coolant leak since i acquired this 99 4.0 ohv 4x4 a year ago.

I notice low coolant by a lower level in the overflow bottle. This first time I noticed it, it was empty cold, but the radiator was full.

I went through a big jug of coolant in 8-9 months

After a pressure test was inconclusive, I decided to just monitor the issue and not worry too much about it. The truck has sat in the same spot for two weeks, and I noticed a saucer sized moist spot on my driveway. It was directly under where the lower radiator hose meets the radiator. I looked at the hose and saw a trail where coolant dripped down the length of the hose and then dripped to the ground. The part of the hose where it meets the water pump was darkened, as though it was wet.

I decided to start the truck and let it get up to temperature and see if I could see anything, I smelled a hint of coolant being blown by the fan, but could not spot a leak, or a "squirt" like when you do a pressure test and find a pinhole. I also watched the thermostat housing and thought maybe i would see a drip or bubbles but saw nothing.

Now most of the evidence is gone, rain washed away the coolant on the drive way, and running the engine looks like it had the fan blow the hose dry.

So, my question, what can cause a slow coolant leak around the front of the engine while the vehicle sits ?

You seem to have pretty much diagnosed it - leaking water pump, or at least the connection to it.

Degraded hose. Possibly the weep hole on the water pump.

I decided to try and confirm this with UV dye. the only thing I found was that it was leaking around where the block and the water pump meet, it was only visible from underneath. I am trying to work up the courage to tackle this repair. I know it sounds silly but I have not tried to replace a water pump before. The tricky part looks like removing the fan.

I did find something else I know I can not do my self, the front diff is leaking around where the axle meets the center part of the diff.