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RPM's and tell-tale

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My 35HP Mercury is squirting water from the tell-tale tube outlet when the engine is warm and in idle. However, if I increase the rpm's just a bit the water stream is there. Is this a sign that the water pump (impeller) is failing or are there some other possible causes? What is the impeller made of (material) and how long do they last? Is it easier to replace the whole water pump housing with the impeller inside from the factory or is the impeller replacement a piece of cake? This motor has not seen any muddy or weedy lakes. Thanks.

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welcome to FM/HSO sugarbeet!

General recommendations are to replace that impeller every 3 years or so.

Some of the older outboards actually do spray at idle so you can't just make a judgment based on that alone if it's an older outboard.

If it hasn't been changed in several years, then it's time.

It's a job a do it yourselfer can tackle, but maybe not classified as a "piece of cake". There are some specific steps that need to be followed.

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Thanks...it's a 1987 model--Mercury 35HP. The impeller has never been replaced. Why does the water stream increase with rpm's? Seems like if there is impeller wear speed would be a non-factor.

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That spurting is a sign for you to replace the impeller.

There is no reason or short cut by replacing the entire waterpump. They are bought and installed in pieces. Those parts are an upper and lower housing, a stainless steel plate, and upper and lower seals, a key, and a gasket. Note: if you start your outboard without a water supply you'll create a lot of heat inside the water pump and burn the impeller and damage the housing and plate. If you have water or milky oil in your lower unit you'll replace the upper and lower seals in the water pump along with the new impeller.

The impeller is rubber, its goes through the driveshaft and the reason your seeing a better stream at higher RPMs is because the impeller is turns at higher RPMs as well. Its simply pumping more water.

Picture of an impeller.


If you plan on doing this yourself, your library will have a manual or better yet buy one.

If you need seals too you might want to have a shop do this for you.

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