Re: Volvo Engine Cooling System
There is an air leak problem somewhere from the raw water pump itself to the seacock. Number of possible causes: the pump itself wears on the back and front faces so if a marginally smaller than normal impellor (say a pattern one) was fitted, or maybe a jubilee on the pipe to the seacock wasnt tightened properly, or maybe a pipe was bent and cracked.
All you need to do it to start off at the seacock and carefully go through every inch of the system up to and including the pump itself. Replace the impellor if nothing else works.
this post is a personal opinion, and you should not base your actions on it.