Water pump problem

I have a 94 40hp 4 cly Mercury. The water pump was pumping fine when I put it up for the winter got it out the other day to make sure everything was working before I took it out on the water and it wasn't pumping water. I replaced the impeller and put it back together started the motor and It began to pump a steady steam till I shut the engine and restarted it then it was back to not pumping water. Only thing coming from pee hole is exhaust. I took lower unit back off check to make sure impeller was installed correct, back flushed water through engine and water tube, checked for blown head gasket and everything checked out ok. Any one have any ideas to what is going on?

Took the cylinder cover off found pieces of the old impeller where the water comes in cleaned all that out now it's pumping fine. Maybe this will help someone else with this problem!