I have a 2001 Discovery with 90k. Runs great. For a long trim, 4-5 hours and then return home in 2 days the anti freeze level stays perfect. Around town I add just a little every other week or so. Why?

 

You have a small leak, perhaps around a hose, or the coolant cap; at eleveated temperatures (slow drivng) the coolant is going somewhere. You can rent/borrow a coolant pressure tester from parts store. Pump up to 15 PSI and let it sit for 30 minutes then look for leaks.

 

Cant one leave the car running until the pressure builds up then identify area?

 

An easier way is to rent/borrow a coolant pressure tester at parts store, pump up system to 15 PSI, drink a few cold ones, then look for leak. Cold ones optional, but this time of year, it sounds like a good idea.

 

Don't put off pressure testing the system before you experience a complete coolant failure.