XJ Coolant Leak

While driving in traffic, I heard my electric fan kick on. This is unusual for my Jeep even in traffic. I looked at the gauges and saw the temperature was running much warmer than usual.

I switched on the heater and rolled down the windows.  The temperature started to drop slowly. As soon as I started moving again, the temperature dropped back to normal.   But each time I stopped, The temperature would rise quickly.

I managed to make it home without a major overheat.  I felt the radiator cap and it was cool to the touch.  This indicated to me that I was low on coolant.

I looked for leaks and did not see any.  I still park over a pan since my Jeep used to leak oil before the replacement.  There has been no evidence of coolant in the pan. I popped the cap and sure enough, the level was low.

I had to add quite a bit of coolant so it was very low.  But I still did not see a leak. I decided to try driving to work again the next day.

After another trip, the overflow bottle was empty and the level in the radiator was down slightly.  I knew I had a leak but there was still no visible evidence.

I got out my Stant Radiator tester. https://amzn.to/387YwvI First I checked the cap. https://amzn.to/2P7VEXe  It relieved at the set pressure of 13 psi and seemed to hold below that.  

Next, I connected the tester to the radiator. https://amzn.to/2rhcdbl As soon as I started pumping I heard fluid hitting the pan under the Jeep.  At first glance, I thought it was the lower radiator hose, but on more careful inspection, I saw it was coming from the water pump weep hole.
Since it only leaked under pressure, I was not seeing the evidence.  I removed the leaking water pump https://amzn.to/34P1x2d and swapped it for a new one under warranty.  I was glad I caught it before I overheated my new engine.