For quite some time now the water level in the cooling system of my 300E drops gradually. I've been topping it with about 0.6 litres of water every 2-3 days. I've taken the car to a mechanic who, after thorough investigation, could not discover any leakage point - I've even poured some colorful coolant to see if that could help detect the leak point (if any) - but no success.
* What could be the possible causes of this problem and what solution shall I apply?
* Could water be evaporating from the cap - the temperature guage never gets above 90 degrees
Thanks in advance for your suggestions.