Winterizing Boat Generator: Onan and Kohler Marine Generator Storage Guide
Winter’s knocking on the door, and your boat generator isn’t going to winterize itself. Whether you’ve got an Onan humming under the deck or a Kohler keeping the lights on, proper storage can mean the difference between firing up smoothly next spring or spending hundreds on repairs. Let’s walk through everything you need to know about putting your marine generator to bed for the cold months—without the technical headaches.
Why Winterizing Your Marine Generator Actually Matters
Here’s the deal: Marine generators aren’t like your car engine. They sit in damp, confined spaces where moisture loves to throw a party. When temperatures drop, that moisture turns into your generator’s worst enemy—causing corrosion, fuel system gunk, and electrical gremlins that’ll haunt you come spring.
I once skipped winterizing my Onan because “it’d probably be fine.” Spoiler alert: It wasn’t fine. Cost me $600 in carburetor work and a week of cursing myself. Don’t be that person.
What Happens When You Skip Winterization
- Fuel degradation: Gas turns into varnish in about 30 days
- Corrosion city: Moisture attacks metal parts like nobody’s business
- Battery death: Cold kills batteries faster than you’d think
- Seal damage: Rubber seals dry out and crack without use
The Essential Pre-Winter Generator Checklist
Before we dive into the step-by-step stuff, let’s talk about what you’ll need. Nothing fancy—just some basic supplies that’ll save you major headaches.
Materials You’ll Actually Use
| Item | Why You Need It | Brand Doesn’t Matter |
|---|---|---|
| Fuel stabilizer | Keeps gas from turning into sludge | Sea Foam, Star Tron work great |
| Engine fogging oil | Protects cylinder walls from rust | Any marine-grade option |
| Fresh oil/filter | Old oil holds contaminants | Check your manual for specs |
| Battery tender | Maintains charge over winter | Automatic ones are foolproof |
| Antifreeze (propylene glycol) | For water-cooled systems | Must be marine-safe |
Pro tip: Don’t cheap out on fuel stabilizer. The $10 bottle could save you a $500 carburetor rebuild.
Step-by-Step: Winterizing Your Onan Marine Generator
Onan generators are workhorses, but they need some TLC before storage. The process takes about an hour if you’re not rushing.
Step 1: Run It With Stabilizer
Start your generator and let it warm up for 5-10 minutes. While it’s running, add fuel stabilizer to your tank according to the bottle instructions (usually 1 oz per gallon). Let the generator run another 10 minutes so the treated fuel circulates through the entire system.
This step is crucial—you want that stabilizer coating every inch of your fuel lines and carburetor.
Step 2: Change the Oil and Filter
Warm oil drains better and takes more contaminants with it. With the generator still warm (but turned off), drain the old oil completely. Replace the filter while you’re at it. Fresh oil for storage means you’re not letting acidic, dirty oil eat away at internal parts all winter.
Step 3: Fog the Cylinders
Remove the air filter and spray fogging oil into the air intake while someone briefly cranks the engine (don’t start it fully). You’ll see smoke—that’s normal. The fogging oil coats cylinder walls and prevents rust from forming during storage.
And yeah, it smells weird. Open some hatches.
Step 4: Protect the Cooling System
For raw water-cooled Onans, you need to flush the system with fresh water, then run antifreeze through it. Close the raw water intake, disconnect the intake hose, and place it in a bucket of antifreeze. Run the generator until you see antifreeze coming out the exhaust.
Freshwater-cooled systems? Check your coolant levels and top off if needed with the right 50/50 mix.
Step 5: Battery Care
Disconnect the battery cables (negative first) and clean any corrosion. Either remove the battery for indoor storage or hook it up to a battery tender. Cold batteries lose charge fast, and a dead battery can freeze and crack.
Kohler Generator Winterization: What’s Different?
Kohler marine generators follow similar principles but have a few quirks worth mentioning.
Kohler-Specific Considerations
Most Kohler units use electronic fuel injection instead of carburetors, which actually makes fuel stabilization more critical. Modern EFI systems are sensitive to varnish buildup in fuel lines and injectors.
Key difference: Many Kohler generators have a “prime” function you should run after adding stabilizer to ensure treated fuel reaches the injectors. Check your owner’s manual—it’ll spell out the exact procedure for your model.
The oil change process is basically identical to Onan, but Kohler recommends synthetic oil for certain models. Again, your manual is your friend here.
Don’t Forget the Exhaust System
Kohler’s raw water-cooled models can trap water in the exhaust elbow. After running antifreeze through, remove the drain plug on the exhaust elbow (if equipped) to ensure no water’s hiding there to freeze and crack something expensive.
Common Winterization Mistakes (Learn From My Pain)
Using Automotive Antifreeze
Marine systems need propylene glycol antifreeze, not the ethylene glycol stuff from your car. The automotive version is toxic and can damage rubber components. Yes, they’re different. Yes, it matters.
Forgetting to Label Things
Take a photo or label what you disconnected. Future you will thank present you when reconnecting everything in spring.
Skipping the Test Run
After winterizing, do a final brief test run to make sure everything’s circulating properly. Just a couple minutes—you’re not trying to get it hot again.
Storage Environment Tips
Where you store your boat matters almost as much as how you prep the generator.
- Ventilation is key: Moisture buildup accelerates corrosion
- Cover but don’t seal: Use breathable covers, not plastic tarps that trap condensation
- Pest prevention: Mice love generator compartments. Consider rodent repellent
- Check periodically: If possible, peek in monthly to ensure no leaks or pest activity
Spring Startup: Quick De-Winterization Overview
When warm weather returns, you’ll need to reverse the process:
- Reconnect and charge batteries
- Flush antifreeze from cooling systems (run fresh water through)
- Check oil levels
- Inspect belts, hoses, and connections
- Run the generator under no load for 10-15 minutes
- Gradually add electrical load to test systems
But that’s a conversation for another day when you’re not staring at snowflakes.
FAQ
Q: How long can I store a marine generator with stabilized fuel?
A: Most fuel stabilizers are good for 12-24 months, but check the product label. For longer storage, consider draining the fuel system completely instead.
Q: Can I just drain all the fuel instead of using stabilizer?
A: You can, but it’s trickier than it sounds. You need to run the generator until it dies to empty the carburetor/fuel lines completely. Plus, storing tanks empty can invite moisture and rust.
Q: Do I need to winterize if I’m in a warm climate?
A: If you’re not using the generator for 30+ days, yes—fuel stabilization is important regardless of temperature. You can skip the antifreeze step if freezing isn’t a concern.
Q: What’s the biggest mistake people make?
A: Honestly? Putting it off until the last minute or assuming “it’ll be fine.” Generators are expensive. Treat them right.
Q: Should I run my generator periodically during winter storage?
A: It’s not necessary if you’ve winterized properly, and actually risks incomplete warm-up cycles that cause moisture buildup. Better to do it right once than halfheartedly multiple times.
There you have it—everything you need to tuck your Onan or Kohler marine generator in for winter without drama. It’s not rocket science, just attention to detail and an hour of your time. Your spring self will thank you when that generator fires up on the first try.
