Outboard overheating: worn water pump impeller

Applies to: All outboards (Yamaha, Mercury, Honda, Suzuki, Tohatsu, Evinrude, Johnson), All years

Symptoms

  • Weak or no tell-tale (pee hole) stream
  • Overheats, especially at high RPM
  • Overheat alarm or limp mode on newer motors

Why it happens

The rubber impeller vanes wear, crack or stiffen, especially after running dry or sitting, and can no longer push cooling water. Yamaha recommends replacement yearly or every 100 hours.

How to fix it

  1. Run on muffs or in water and watch the tell-tale stream.
  2. Rule out a clogged tell-tale first: pull the hose inside the cowl and blow the fitting clear.
  3. Drop the lower unit and open the water pump housing.
  4. Replace impeller, liner, gaskets and O-rings (kit $30–80).
  5. Seat the drive key correctly and install the impeller with vanes facing the right way; the manual shows orientation.

Tools and parts

  • Flush muffs
  • Water pump / impeller kit
  • Socket set
  • Marine grease

What the factory service manual covers

Cooling system: impeller procedure, housing torque specs, lower unit removal. The full procedure, specs and wiring diagrams are in the factory service manual for your machine.

Get the factory service manual

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