Yamaha cooling issues.

[whydoihateboats]

I have a 1992 Rinker w/a 4.3 Yamaha, I changed the raw water pump impeller, the housing looks great. With the output hose off the pump and the muffs hooked up I'm not pumping water. I removed the intake hose from the plastic pipe and put the hose down it, started the boat and it pumps great. If I put the hose water down the plastic pipe I get water out the intake vents on the lower unit but if I put the muffs on I get nothing flowing up and out of the plastic pipe. Any ideas on where to go next. Thank you!