I have a 2001 Discovery. It runs great. On a trip the over flow tank stays perfectly. Around town I check the overflow tank and for a period of two weeks or so I add a cup or so of anti freeze. Car never has had an over heating issue.
Can I put a sealer in or will I need a head gasket?
Thanks.

 

NO, rent or borrow a cooling system pressure tester and find the leak

 

1. Overheating to many is defined by heat gauge going very high, or red light coming on. It actually begins much sooner, the factory gauge points at 50% from about 130-240F. A scanner will tell the real temps.

2. Loss of coolant is not always head gaskets. You can loan/reant a pressure tester from the parts store, pump up cooling system to 15 PSI, and let it sit for 30 minutes to reveal the leak.

3. Sometimes the coolant cap will vent while driving at less than 15--18 PSI. So coolant evaporates before you get home. A paper towel zip tied around overflow line can detect this.

4. Putting in a sealer will close small spaces related to leaks. It will also at the same time find small spaces in sludged radiators and heater cores, which are working now, but slightly restricted. All brands will treat these spaces as well, closing them off, so you can have a much bigger problem.