I suspect a new thermostat will fix your problem.. You shouldn't need to add or subtract coolant. Fill it to the LOW line when cold and it should work fine. Thats why the expansion tank is there. Also you should never run just water.. A proper anti-freeze mix in important.. Zerex G-05 or MB antifreeze is what you must use.

__________________
Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.