When I start my boat on muffs or when it first starts up in the water i am not getting any water out of the exhaust. I have to rev up the motor high to get it started, then it works fine. It has a new impeller in it. I guess I could of put the hoses on backwards, but it was doing it before the change.

Any ideas on what else to check? Could this be caused by the oil cooler blocked? It seems to be running at normal temp.

Thanks

What is your setup .merc alpha,bravo ..volvo,Other ?Year model?

It takes about 1/2 a second to trash a brand new impeller. Not saying it's bad but worth a look. If the old impeller was torn up? There might be a piece lodged someplace in the system and restricting water flow since you said it's slow to prime even on the hose. They all seem to prime slow at first start. If you don't rev it, will it eventually pump water on it's own? I'm assuming there is no raw water pump? Just the pump in the drive? I doubt the oil cooler has anything to do with it or you probly would be having some overheating issues. I'm also assuming no strainers? I/m sticking with a bad impeller or pump housing is worn, or blockage someplace. Just my 2cts worth.

1996 BravoI, 454 Mag. It has the water pump on the back of the fuel pump.

I have worked on a few boats and never had a problem with running them on muff's, except for one buddys boat...Its a searay cruiser with a 350mpi and bravo3 drive. It has the water pump on the side of the engine and not in the drive. The start of this year we fired it up and let it idle and it would never start to pump water from a garden hose with muff's. So I replaced the raw water pump and fired it up again..same thing will never prime at idle. I reved it to 3500 and back to idle and it primed and was ok after that. For some reason it would not draw the water in at idle running it on the muff's. I think its kinda the nature of the beast...Goodluck
Erik

All Bravo drives have the seapump bolted to the side of the engine and run off a belt.
It always takes about 3-5 seconds to get water out the exhaust after starting the engine. How long are you talking before you get water out the exhaust???
Do not use Sierra Impellers. They do not like to prime and do as you describe quite often.