Have just fitted a new impeller to my Yamaha 175 v6 as after leaving engine over winter when I restarted I last week it would pump water one minute then next time would not .Took out old impeller expecting it to be damaged but nothing wrong with it .Fitted new impeller and same problem infact is now worse as will now only pump water if I use muffs and wont draw it from the tank .have tried putting hosepipe directly on riser pipe and water circulates out of pee hole at high pressure (mains water pressure) so assume engine is not blocked any ideas or should I have fitted pump repair kit and not just the impeller.