Sierra 18-3251 Water Pump Kit | Impeller, Wear Plate

Your engine's raw water pump is the heart of its cooling system, and the impeller inside it is the one part guaranteed to wear out. The Sierra 18-3251 Water Pump Kit gives you the three components that take the beating: a fresh impeller, a new wear plate, and a new upper plate gasket. Refresh them on schedule and your engine keeps pulling a strong, steady stream of cooling water every time you run. Let them go too long, and you're gambling with an overheat that can cost you a powerhead.

This is a service kit, so it covers the parts that actually degrade. The impeller is the rubber vane wheel that moves water. Over a season of running it takes a set, the vanes stiffen, and the corners start to crack. The wear plate is the flat surface the impeller rides against inside the pump housing, and a scored or grooved plate will chew up a brand new impeller in short order. The upper plate gasket seals that plate so water goes where it should instead of leaking past. Replacing all three together is the right way to do the job once and do it right. Keep in mind this kit is the wear-parts refresh. If your pump also needs a new housing, cover, or the lower seals, those are separate items, so check what your particular pump calls for before you tear into it.

A few things we've learned doing these. Note which way your impeller vanes curl before you pull the old one, because the new impeller has to bend the same direction the pump turns. Put it in backwards and it'll shred on the first start. Lube the new impeller with soapy water or a little glycerin, never petroleum grease, since petroleum swells the rubber and ruins it fast. Check that the wear plate sits perfectly flat with no warp. And when you close the pump back up, snug the cover screws down in a star pattern so the plate seats evenly and doesn't distort.

One more tip that saves grief later. Scrape every bit of the old upper plate gasket off the mating surfaces before you set the new one down. Old sealant or a torn scrap of the previous gasket is the number one reason a freshly serviced pump still weeps. Take a minute with a plastic scraper and get it clean. While you're in there, look over the pump housing itself for cracks or heavy pitting. This kit renews the wear items, but a housing that's damaged needs its own attention, and it's better to find that on the bench than at the ramp.

When should you do this? Most of us pull the pump at least once a season, or roughly every hundred hours, whichever comes first. If you've run through sand or a muddy shallow, do it sooner, because grit is brutal on an impeller. And if your telltale stream has gone weak or you've seen the temp needle creep up, don't wait, that's the pump telling you it's tired. It's a couple hours of work on most outboards and sterndrives, and it's cheap insurance against a very expensive afternoon.

Sierra builds these to meet or exceed the original equipment part they replace, so you're not giving anything up by going with the kit instead of the dealer part. Match your engine's original water pump part number to the Sierra 18-3251 reference before you order, and you'll know it's the right fit. It's the kind of maintenance part every boater should keep a spare of on the shelf, because the day your impeller finally lets go is never a convenient one.