diesel boat in water featured image for winterize marine diesel article


We’ve had a fantastic summer, but the weather feels like it’s gone straight to winter overnight. The time has come – time to winterize your boat. Here are a few tips from the pros to make sure you winterize your marine diesel engine correctly:

1. Treat the Diesel Fuel

Unless properly treated, diesel — especially the newer bio-diesel and low-sulfur fuels — can grow stale and prone to bacteria and fungal infestations while in storage, resulting in sludge and sediment that can plug filters, create starting problems and damage engines. Also, an empty fuel tank invites condensation, and over time this results in water collecting in the bottom of the tank, posing a serious problem for diesel engines. To help prevent any of these maladies, fill up before long-term storage. Then treat the fresh fuel with a high quality diesel additive such as Stanadyne Performance Formula. After treating the fuel, install new primary and secondary fuel filters, and then bleed the fuel lines to eliminate any air pockets.

2. Change the Oil

Used diesel engine oil contains acids and other contaminants that can eat away at metals over the winter. So ditch the old oil now. To change the oil, run the engine for a few minutes to warm up the oil (if the boat is out of the water already, you’ll need to supply cooling water to the engine. Then shut down and drain or pump out the old oil. Change the oil filter and fill the crankcase with fresh oil (per the manufacturer’s specifications). This is a good time to change the transmission oil too.

3. Ensure Proper Drainage

Open all drain plugs to purge the raw-water cooling systems. Plug locations vary by manufacturer, so check your manuals. Use a stiff wire to clear any sediment from drains. Also, bump the ignition to turn over each engine (without starting it) to clear water from the pumps. If your boat stays afloat all winter, you can drain the systems by first closing the seacocks for the raw-water inlets, then removing the inlet hoses and intake-pump covers, as well as all drain plugs. After clearing the raw-water systems, replace all of the drain plugs. If you removed the intake-pump covers, give each impeller a light coat of Vaseline and replace the covers.

4. Prevent Freezing

To avoid freeze damage and fight corrosion over winter, plumb the motor flushers or intake pumps to draw rust-inhibiting propylene-glycol antifreeze from a bucket or tank, and run each engine until the solution exits the exhaust. Not only does this displace any standing water and coat the water jackets and heat exchanger with a corrosion inhibitor, but at the same time it distributes inside the engine and transmission a coat of the fresh, clean oil that you put in earlier to help prevent internal rust during storage.

5. Do an Inspection

While you could wait until spring, now is a good time to check zincs, belts and electrical connections, as well as the O-rings on fuel fillers. Also inspect the physical connections at the transom and along the exhaust outlets — lots of water flows through here! Replace, repair and service anything that looks suspect or worn, and you’ll have that much less to do next spring.

6. Seal It Up

Seal the exhaust outlets on the hull and air filters on the engine with heavy plastic and duct tape. This prevents moist air from finding its way into the combustion chambers via open exhaust or intake valves while your diesels are enjoying some well-deserved downtime.