MerCruiser Sterndrive Winterization Guide

A sterndrive is a bigger job than an outboard, and the reason is simple. An outboard drains itself when you tilt it down. A MerCruiser holds water in the block, the manifolds and the hoses, and water left in an engine block that freezes will crack it. That is the expensive failure this whole procedure exists to prevent.

Running a Mercury outboard instead? That guide is here. For the general walkthrough, start with the boat winterization guide.

What your MerCruiser needs

Fuel first, with the engine running

Add stabilizer to a nearly full tank and run the engine long enough to pull treated fuel through the water separating filter and into the injectors or the carburetor. Then change the fuel filter. Leave the tank close to full so there is less air space for condensation.

Oil and filter while the engine is still hot

Do this before you drain the cooling system, while everything is warm and the oil flows. Used oil holds acid and water, and leaving it in the pan for five months is how you get bearing corrosion that was completely avoidable.

MerCruiser gas engines want a Mercury 25W-40 FC-W marine oil in most applications, plus a new filter. A fluid extractor is worth owning if your engine does not have a drain pump.

Now get the water out. This is the step that matters

Know which system you have before you start. A raw water cooled engine pulls lake or sea water straight through the block. A closed cooling engine circulates coolant through the block and uses raw water only through a heat exchanger. Both have raw water somewhere, and both can freeze.

  • Open every drain point. On most MerCruisers that means both sides of the block and both exhaust manifolds, plus the hoses between them. Newer engines have a single point drain system, but the drains still have to actually flow. If one does not run, poke it clear. A plugged drain that you assumed was empty is the classic cracked block story
  • Once it is drained, run non-toxic antifreeze through the raw water circuit until it comes out the exhaust at full strength. Plan on three to four gallons
  • Do not put automotive green antifreeze in a raw water circuit. Pink non-toxic is the correct product

If you have closed cooling, check the coolant in the closed loop too. It has a service life and a freeze point, and it is worth testing rather than assuming.

Fog it, then new plugs

Fog the engine through the flame arrestor until it smokes and stalls, then install new plugs. In that order. Fogging fouls whatever plugs are in the engine, so a new set installed first is already spent by spring.

The drive

The drive is a separate job from the engine and it does not get skipped. Alpha and Bravo drives have different lube requirements and different reservoirs, so check the book for yours.

  • Drain the drive, read what comes out, and refill with the specified gear lube. Milky lube means water past a seal and that is a repair to book now
  • New sealing washers on the fill and vent screws. Cheap parts, most skipped item on the list
  • Top the gear lube monitor bottle
  • Grease the U-joints, the gimbal bearing and the steering with the right grease
  • Look hard at the bellows. Cracks, soft spots and chafe are cheaper to find in October than to discover when the bilge alarm goes off
  • Store the drive in the full down position so it drains and the bellows are not stretched

Impeller

The sea water pump impeller is a wear item on a schedule. On an Alpha the pump lives in the drive, on a Bravo it is engine mounted, so the job is different depending on what you have.

Fall or spring both work. The argument for fall is that you are already into the drive for the gear lube on an Alpha, and that an impeller that sits all winter with its vanes bent in one direction takes a set. If it was marginal in October it is worse in May, and you find that out with a temperature alarm.

Buy the full kit rather than the bare impeller. The wear plate, housing, gaskets and seals are why people do this job twice.

Finish it

  • Check the anodes on the drive and the trim tab. More than half gone means replace now
  • Batteries off the boat, terminals cleaned and coated, on a maintainer
  • Pump the bilge dry and leave the plug out
  • Moisture absorbers in the cabin
  • Cover it with vents, and put it on stands if it is coming out of the water

If you are not sure which drive you have or which lube it takes, call us at 631-722-5700 or send us your model and serial number. Always check your MerCruiser owner's manual for drain locations, capacities and service intervals for your specific engine and drive.