Read this before you start: drain points, antifreeze quantities, gear lube capacity and torque values are specific to your engine's exact model and serial number. This guide covers the method every winterizing job follows. For the numbers, use your engine and drive's own service manual. Guessing at a drain point on a block you haven't verified is how engines crack over a Michigan winter.
Why winterizing matters here
The goal of winterizing is simple: no water left anywhere that can freeze and crack a block, manifold, heat exchanger or pump, fuel that won't gum up over the winter, and a boat that's dry and ventilated when you open it up in spring. Lake St. Clair boats sit through months of hard freezes, and a block that wasn't fully drained or protected can crack in a single cold night, turning a routine service into an expensive repair.
Safety first
Work in a ventilated space, since you'll be handling antifreeze and fuel. Let the engine cool before draining fluids or changing oil. Disconnect the battery before doing any electrical work or long-term storage. If the boat is on a lift, trailer or blocks, make sure it's properly supported before you're underneath or reaching into the drive.
Step 1: Change the oil and filter while the engine is warm
Run the engine until it's warm, then change the oil and filter. Oil changed warm carries more of the acids and condensation out of the crankcase with it than oil changed cold, which is exactly why this step happens before layup rather than being saved for spring.
Step 2: Winterize the raw water side
Gas sterndrives are raw-water cooled, meaning lake water runs directly through the block and out the exhaust. You have two options: run non-toxic propylene glycol antifreeze through the system on a set of flush muffs until it runs pink from the exhaust, or physically drain every block, manifold and cooler drain point on the engine. MerCruiser blocks in particular have several separate drain points that aren't in the same place from one model to the next. This is the step where VERIFY matters most: find your engine's own drain map before deciding which approach to use, and don't assume antifreeze alone reached every pocket without confirming the drain points too.
Step 3: Service the gear lube
Drain the sterndrive's gear lube into a clean pan and look at it before you refill. Milky or cloudy lube means water has gotten past a seal, which is worth investigating rather than ignoring going into winter. Refill with the gear lube grade your drive calls for and top the monitor bottle to its line. Gear lube capacity differs between drive models, so check your drive's own spec rather than guessing at a quantity.
Step 4: Stabilize the fuel
Fill the tank most of the way (leaving some room for expansion limits condensation over winter), add a fuel stabilizer, and run the engine briefly to circulate the treated fuel through the whole system, not just the tank. If your engine maker calls for fogging the cylinders on shutdown, do that as the last step before the engine goes quiet for the season.
Step 5: Fresh water, head and bilge
Drain the water heater and tanks, then pump non-toxic propylene glycol antifreeze through the freshwater lines, faucets, shower and the head until it runs pink at every outlet. Empty and treat the holding tank. Clean and dry the bilge, and it doesn't hurt to add a little antifreeze to the bilge's low point too.
Step 6: Batteries
Charge the batteries fully. From there, either disconnect them for the winter or leave them on a smart charger built for long-term maintenance, and clean the terminals while you have access.
Step 7: Final check
Before you call it done, confirm antifreeze actually ran pink through every water path you touched, engine, freshwater lines and head. Make sure every drain you opened is closed and secured again. Check that oil and fuel are handled, batteries are set for winter, and the boat has some ventilation so trapped moisture has somewhere to go instead of turning into mildew.
Where this gets more complicated
Diesel engines and generators are winterized differently from gas sterndrives, with antifreeze going only on the raw-water side while the closed coolant loop stays on its rated coolant. Jet drives and personal watercraft each have their own flush, drain, fog and stabilize sequence that varies by maker. If your boat is any of these, or if you're not fully comfortable finding your own drain map, that's a reasonable place to stop the DIY approach.
See winterizing in the water vs. on the hard for timing this around your haul-out, or the haul-out inspection checklist for what else to look at while the boat's out.
Or have Brad do it
Text the year, make and engine, or a photo of the engine, and your haul date.