Pairing hoses with fittings must be reliable.
Check the tightness of the hose clamps. If the fastening is not secure, then replace the terminal clamps with screw ones.
If the coolant level drops and no leak is found, check the system with a warm engine. Start and warm up the engine. Let it run at idle until the fan turns on. Check for fluid leakage around the coolant pump.
Significant loss of coolant and the presence of oil in the coolant, as well as white clouds in the exhaust gases, indicate damage to the cylinder head gasket.
Caution: It is often difficult to locate a leak. In this case, a pressure test is recommended, see section «Checking the cooling system by pressure». In this case, you should also check the safety valve located in the cap of the expansion tank.