water pressure

In order to build pressure in the cooling system, the radiator cap must be in good operating order. After the engine is at operating temperature, the system should build up to 16 psi. On a rare instance, I have seen a water pump turn adaquately, but the impellor was slipping in the shaft.

There is no way to tell if the impeller is slipping on the shaft without removing the water pump to inspect. I have seen it on some V8 Jaguar and some 4 cylinder Audi/VWs.