The water pump impeller of any outboard motor needs to be serviced on a regular basis, a task that if overlooked can lead to big trouble. Most outboard maintenance schedules call for inspection of the impeller every 100 hours, or once a year. And once you’ve paid a tech to drop the gearcase and look at the impeller, you might as well spend another £25 and get a new one installed. The entire job is about £100 of cheap insurance.

The torn-up blades of this inboard impeller are not going to move much water.

The impeller itself is simply a series of rubber vanes molded around a hub. The vanes are flexible, and the hub rotates on an eccentric within the pump housing, which has a stainless steel liner. The tips of the impeller vanes can wear out from simple use, but if the motor is often run in silty or sandy conditions, this abrasive material can accelerate impeller wear. The rubber material can also get stiff and brittle, a common problem on motors that sit unused for several seasons. Then the vanes can “take a set” in one position and not flex back. The other impeller-killer is heat. Water lubricates the pump, and if it’s run dry the impeller can be ruined in just seconds.

Note I said impeller service requires “dropping the gearcase.” For most of us checking the impeller is not a do-it-yourself project, which is probably why so many impellers are neglected until they fail.

“I think the most-common cause of impeller failure I see is simply old age,” says Dan Jansen, the lead certified Mercury technician at Mr. Marine. “Even with our short season, I find a lot of impellers that are too worn at the tips to pump well, or are really stiff. We make changing the impeller part of our off-season lay-up program, because if it fails you can have much more expensive problems.”

This Mercury outboard water pump was run dry briefly, and the impeller literally melted within the housing. You can see the imprint of the impeller on the back. In this case the entire water pump was replaced.

When the impeller fails, the pump can’t pump and the engine will overheat. I’ve seen old outboards that got so hot the paint burned off the powerhead. Most newer motors have a warning horn that sounds when the motor gets hot, and a program that cuts engine rpm to try and protect the powerhead, but Jansen thinks that often may be too little, too late.

“If they are running wide-open, by the time they hear that horn and react, they have a scuffed piston, especially if it’s a two-stroke motor,” said Jansen, “ And even if the motor is OK, now they are out in the middle of the lake and the day is shot, at the very least.”

A ruined impeller can also be caused by running aground, says David Greenwood, product planning manager at Suzuki Marine.

“You can suck a lot of abrasive trash into the pump if you run the gearcase into a sand bar,” says Greenwood. “I’ve seen cases where the water intakes were just packed to the point that water could not even get to the pump. If you get in that situation, it’s a good idea to have the impeller checked just as a precaution.”

The impeller to the right is new. The two to the left are old, with vanes that are worn, hard and curled over from sitting idle for years in an older motor.

Jansen and Greenwood agree that starting the engine “dry,” that is, with no water supply to the pump, is instant death to the impeller.

“A few years ago Suzuki actually did a study to see how long an impeller would last if you started the motor dry,” said Greenwood, “and it was about 20 or 30 seconds.”

Jansen showed me a Mercury outboard water pump housing he removed this season that is coated with melted rubber (see photo #2, above).

“The boat owner says he doesn’t know what happened, but I know he started the motor in his driveway, just because he likes to hear that two-stroke bark,” said Jansen. “This one got so hot you can see the stainless steel liner has actually recessed a little into the housing. The whole thing was melting.”

Greenwood says he’s seen boat owners who give the motor a quick bump with the starter on the launch ramp, just to make sure it’s ready to go when they get in the water, and others who think they need to start the motor after pulling the boat out of the water to drain it. Both are big mistakes that will ruin the impeller.

“Another issue is owners think they can start the motor if a hose is hooked up to the flush port,” says Greenwood. “That pushes water through the powerhead and exhaust, but very little gets to the pump. We tell owners to always use a set of water muffs on the gearcase if they want to start the motor with the boat on a trailer.”

If an impeller fails dramatically, simply installing new parts may not be the end of your trouble.

“If the impeller really comes apart, there’s a good chance that little bits of rubber can get lodged in the water tube and restrict flow to the powerhead,” says Greenwood. “You might notice a weak stream from the water pilot out the side of the motor, and that could mean that the technician did not check for impeller debris. If that’s uncorrected, the motor could overheat.”

You’ve been warned. Replace that impeller at the end of this season.

plueddeman-head-shotCharles Plueddeman is's outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.

Written by: Charles Plueddeman
Charles Plueddeman is's outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.