I have a 15hp, and I am wondering if its air cooled or water cooled? I got this motor from my uncle who was going to toss it. It was able to get it running, but there is no water, just air coming out the back! I don't want to take it out on a test run on the boat, just incase the water pump isn't working.
There's an impeller that is located in the lower unit. Pretty much every outboard out there has it located there. You'll have to remove the lower unit to replace it. While you have it off it's a great time to flush the lower unit and fill with new gear lube. Don't run the motor any more until you have a new impeller. You can get the impeller for less than $20 on here.
Some guys replace the impeller every year, just to make sure their motor is always cooled. Considering the cost of a new motor, the impeller is a small price to pay.
The easy way to tell is if the motor doesn't have a steady stream of warm/hot water when running then it's not pumping. Spitting water doesn't count either, you need a steady stream.
I count this in the "cheaper to replace this than the whole motor" category, and change it whenever there's ANY doubt. For such a simple job there's really no excuse to ever burn up a motor due to a bad impeller. jmho