Impeller Change on Merc 8.1, no water flow

[JustRight]

I changed my impellers about 3 yrs ago using this thread as my roadmap and, as an FYI, I used a fair amount of dish detergent to lube up the new impellers to get them seated into the pumps. Anyway, when I started the engines, it took a little longer than usual but they pumped out a fair amount of foamy dishwater once they primed themselves. I did not have scoring on my pumps, so I recognize that those with scored pumps can have a harder time getting that initial prime to pick up in the raw water pump. I have to do the impellers again next spring and I intend to lube the new ones up the same way...it worked great for me.

 

[Sanibel Life]

Great job on a nice and necessary thread. Many of us wrench turning boat owners see the impeller replacement as annual maintenance to keep from throwing impeller chunks and all that fun.

Positive displacement pumps like these are self priming, which means they pump air or water..or ketchup. They pump the exact amount of water (or air) that fits between the vanes. The only way to get them to pump more is to spin them faster, but it is always proportionate to the vane size. We do this by increasing RPM.

They do require a perfect seal to work perfectly. Not to work, but to work perfectly. If the seal is not perfect you can get intermittent problems drawing water. Sometimes it works perfectly, sometimes it doesn't. This sounds like your situation.

When replacing yours, check for scoring on the brass bottom and top of the impeller housing. Scoring is common and occurs when abrasives in the water (sand, barnacles, fish bones) are carried in the water and get pinched between the impeller and the housing surface. The scratches are circular and look like tiny crop circles, if you will. See the attached picture.

If you see scoring, replacing the housing is a consideration. They can be purchased complete with impeller installed or you can choose the part you need (top (cover) or bottom (housing) only), but with those you will need to buy a new impeller and seal as well.

I recommend changing the pulley at this time if it is rusty. They are cheap, easy and reduce belt wear. However, I admit I am the kind of guy who replaces bolts, nut and uses anti-seize to make easier work next time.

It is easier to do when it is on your bench than upside down in the bilge in the summer heat, but sometime you have to race with what you brought to the track.