I have the S700 Hymer on the Mercedes 410D 1992 chassis. It is overheating when pulling up hills. I was using about a litre of water every time I checked the radiator. I had it in for a service this week and the radiator has been drained and flushed. It was then pressure tested and a small weep of coolant was coming out of the bottom hose. The hoses have been taken off and cleaned and retightened. The system was retested and no leaks. Yesterday I went for a test run and the temp gauge still rose to almost the top of the gauge on steep long climbs. I did notice the fan was not cutting in as it used to do. Much shorter run only when the temperature was well into the top segment of the gauge. It used to cut in at a lot lower temperature. The fan is a viscous fan coupled to the fan belt and running continuously. However on reading the handbook it has a clutch system filled with silicone operated by a bi-metal strip. When the engine is cold the clutch is disengaged and spins at about 1000rpm. When the engine reaches 85c the bi-metal strip expands and the clutch engages and the fan now runs at same speed as the engine. At 21 years I wonder if this clutch is wearing out and that is the cause of the problem. It was noticeable when the fan cut in but we are now noticing that this is not so frequent as it was even earlier this year. I have also noticed that the temperature is varying considerably. With the interior heater fan on full and the radiator fan cutting in the temperature drops very quickly from near the top of the gauge to almost bottom (below the first mark) almost to the thick cold mark. The gauge is divided into thirds and the pointer normally stays in the lower to centre area. The only time I have noticed it going above this before was on the long slow pull over the border into Spain from France fully loaded and towing a Smart Car on a Trailer. The current problem is even when the van is solo and lighter loaded. We have had problems a couple of times with the fan belt tensioner. We had a new one fitted in Spain last winter at a Mercedes dealership. Has anyone any experience of problems with these fans?