I have tested expansion tanks in he past by plugging the return line and using a regulated air source at 10-15 PSI in the outlet. Preferably, this is done under water in a bucket, making small leaks easy to spot and giving some safety against the tank rupturing. Being away from home in Montana, I had limited tools and no access to an adequate shop or an air compressor. In this case it didn't matter. With my thumb over the return line, I blew in the outlet and I watched air and coolant bubbling at the seam inside the cap drain hole. A low-budget pressure test for sure, but my leak was bad enough for this method to be effective and reveal the leak.

On the 4.8 liter engine, there is a drain in the bottom tank of the radiator and another in the engine block. With the key in the ignition and set to the second click (engine light on, but engine off), set the climate control to max temp (84 degrees) and the fan on low. This will open the valves to the heater core and permit the full system to drain.

Refilling the system is a 50/50 mix of coolant and distilled water - about 12 liters in the 4.8i.