Back in December I parked my XJ8 (2004) on a sloped driveway. After a couple of days I saw a coolant puddle (about a cup or two) that seemed to originate from the back of the under body tray (right side). Looking around it seemed to be coming from the valley. I changed the valley hose (very soft but seemed not to be leaking). While I had the intake off I changed the thermostat housing and checked all the hoses. Afterwards all seemed fine. Yesterday I noticed a leak in the exact same spot. I have driven ~3000 miles since the repair and needed to add approximately 1 cup of coolant about 800 miles in (which I assumed was a bad bleeding procedure). The cooling system was down a cup (not enough to trigger the warning). Today I removed the bottom tray and did not see an obvious leak. I proceeded to pressure test the system. It held 20 psi (Harbor Freight test kit) for 1/2 hour and all hose connections were dry. There was a small wet spot on the tray near the right front (under the auxiliary pump). But I do not see any wet spots on the hoses or cooling system components. After changing the oil I put everything back together. Any ideas where I could be leaking from?

 

How old is your auxiliary pump?
It could be leaking, when its running.
Or the extra pressure, from parking nose tilted down.

 

Pump was changed 1 year ago. The puddle formed nose up (showed up under the middle of the car. I believe its leaking while parked. The puddle looked like about a cup or two and formed after being parked for a week. System was about a cup low. With about 3000 miles I would think a running leak would show a larger loss.

I'll be away for a week and a half. When I return I'll try pressuring with the key in the run position and see if that makes a difference.

 

You will definitely need to do a pressure test, likely that it will be hard to spot such a small leak in that crowded area of the engine bay. And while everything is hot it probably evaporates. The coolant reservoir is most likely, as they can leak from the purge hose nipple. It's also possible the upper radiator hose is leaking, or the radiator itself has a small crack in it.

 

While cold it held pressure for 1/2 hour. Reservoir was replace 2 years ago with an OEM unit.