Why it matters

The rubber impeller in the lower unit is the heart of the cooling system. Mercury recommends regular replacement regardless of hours — a dried-out or chunked impeller is the number one cause of overheat calls. If the boat sat for two or more seasons, replace it on spec, don't inspect it.

Symptoms of a failing impeller

• Weak or steamy telltale stream at idle
• Overheat alarm at cruise but OK at idle (or vice versa)
• Temp creeping after extended low-speed running in sandy or silty water

Procedure overview

1. Disconnect the battery and remove the kill lanyard. Shift into forward gear first if the shift shaft requires alignment on reinstall (varies by model — check the service manual).
2. Support the engine, drain nothing — this is a dry job. Remove the lower unit bolts (note the trim tab bolt hiding under the tab on many models).
3. Drop the gearcase, keeping the driveshaft straight so you don't bend the water tube.
4. Remove the pump housing, impeller, wear plate, and gaskets. Note impeller rotation direction before removal.
5. Inspect the wear plate and housing insert for grooves — replace the full kit, not just the impeller. Look for missing vanes; if any are gone, chase them into the powerhead passages or they will block cooling later.
6. Install the new impeller with the recommended lubricant, rotating the driveshaft in the run direction while seating the housing.
7. Reinstall the gearcase, aligning driveshaft splines, water tube, and shift shaft. Torque the fasteners to the value in the service manual.
8. Water-test on muffs before the lake test. Confirm a strong telltale within seconds.

Common mistakes

• Reusing the old wear plate or gaskets
• Forcing the gearcase up with the shift shaft misclocked — it will not shift afterward
• Bending the water tube grommet so the pump output leaks into the midsection
• Running the engine dry "just for a second" — that alone can toast a new impeller

When to walk away

If lower unit bolts are seized into a corroded midsection, stop and quote penetrant time or a shop visit. Snapping a bolt dockside turns a one-hour job into a mess. Same if the driveshaft splines show rust — the customer may have a bigger water intrusion problem.

Safety: battery disconnected, lanyard off, and never spin the flywheel with the gearcase off unless you control the shift linkage.