Winterizing Yamaha Outboard Motors - Here's How

If your outboard starts running hot, don’t panic—but don’t ignore it either. Common causes include a worn impeller, clogged water intake, low oil, or a stuck thermostat. This guide walks you through how to recognize the signs, fix the issue, and prevent long-term engine damage.

What happens if a boat motor is running hot?

When your outboard overheats, it’s more than just an inconvenience—it’s a serious warning. You could be looking at warped components, blown gaskets, and in some cases, complete engine failure. One time, we saw steam pouring from a Yamaha 115 halfway across the bay. Not fun when your only option is paddling.

1. The Water Pump Impeller Wears Out

The impeller is that humble little part that keeps cooling water moving through the engine. Over time, it can crack, split, or get jammed by debris.

Fix it:
Pull the lower unit and check the impeller inside the housing. If the blades look chewed up or stiff, swap it out. Grease the new one before installing, and make sure everything fits snugly before reassembly.

2. Pressure Control Valve Gets Gummed Up

This valve keeps your water pressure steady—but salt, sand, and old grime can clog it up.

Fix it:
Remove the valve and soak it in a marine cleaner. Use a small brush to scrub it clean. If it’s still sticky or shows wear, replace it and reseal the threads to prevent leaks.

3. The Thermostat Refuses to Open

Your thermostat regulates how much cooling water flows through the engine. If it’s stuck shut from deposits or corrosion, the engine will overheat in no time.

Fix it:
Pop the cover off, pull the thermostat, and give it a vinegar soak or hot water test. It should open fully. If not—toss it and replace it.

4. Debris Blocking the Water Intake

You’d be surprised how often the issue is as simple as a wad of seaweed stuck in the grates.

Fix it:
Start by checking the intake screens on the lower unit. If they’re blocked, clean them with a brush. Still no flow? Remove the lower unit and flush the passages with fresh water or compressed air.

5. Internal Water Passages Are Clogged

Mineral buildup can slow down water circulation even if your intake and impeller are working.

Fix it:
Use a descaling solution and flush the system. Connect earmuffs to your hose, run the engine at idle, and circulate the cleaner. Rinse well afterward.

6. The Belt Is Loose or Worn

If your outboard has a belt-driven water pump or other accessories, a bad belt can lead to overheating.

Fix it:
Check for cracks or frays. Replace as needed and make sure the new belt is properly tensioned—not too loose, not too tight.

7. Low Engine Oil

This one’s obvious: less oil = more friction = more heat. And heat is an engine killer.

Fix it:
After the engine cools, check the oil level using the dipstick. Add Yamaha marine oil as needed. Just don’t overfill. And always reseal the cap.

8. Steam Coming Out of the Exhaust

Seeing steam billow out the back? That’s your engine begging for help.

Fix it:
Shut it down immediately. Check for water flow, inspect the impeller, and review the thermostat. Steam usually means something’s stopping the water from doing its job.

FAQ: Yamaha Outboard Overheating Issues

Why is my boat motor running hot?
Could be a clogged water intake, worn impeller, low oil, or thermostat problem. Any of these can knock out the cooling system.

What kind of damage can boat engine overheating cause?
You’re risking warped heads, blown gaskets, cracked blocks—even total engine replacement. Catch it early.

How do I check if it’s overheating?
Feel the telltale stream. If it’s too hot to touch, something’s wrong. Weak or no stream? Time to inspect the intake and impeller.

Real-World Boater Tip

Last fall, we noticed our Yamaha 90HP was idling rough after 20 minutes of trolling. Weak stream from the telltale? Turned out a small piece of eelgrass was clogging the intake. Five minutes with a zip tie and a water flush fixed it. Skipping that check could’ve meant a tow.

Final Thoughts

Routine checks matter. If your Yamaha’s acting hot, stop and investigate—before a small fix turns into a major repair.

When in doubt, refer to your Yamaha owner's manual or call a certified marine tech. If you’re doing it yourself, use OEM Yamaha water pump kits and cooling system parts. You’ll find them all at YamahaOnlineParts.com.

Taking care of your outboard now means less stress later—and more time on the water where you belong.