I had one of those days where everything on the docket needed attention at once — a pressurized lower unit that blew rocks and grease all over me, a Vessel View calibration check, a cowling latch cable swap, and a customer’s boat that lost prime because of an air leak in the fuel line. But the main event was a 100-hour service on a 2006 Yamaha F225, and specifically pulling both thermostat housings to swap in new thermostats. If you’ve never done this before, it looks intimidating with all the hoses, sensors, and brackets in the way. It’s not actually that bad once you know what to disconnect and in what order — and that’s exactly what I’m walking through here.
Why It Matters
Thermostats are a cheap part that controls something expensive: your engine’s operating temperature. Run a stuck-closed or worn-out thermostat and you’re looking at overheating, especially once Florida summer heat starts stacking on top of engine load. This is baked into a 100-hour service interval for a reason — it’s preventative, not reactive. Catching this now is a lot cheaper than catching it after the engine’s already run hot on the water.
Products
- Yamaha OEM thermostats (port and starboard) — the actual part being replaced on this F225; I didn’t get a specific part number or thermostat spec called out in this video, so flag that before trying to affiliate-link it
- 10mm wrench/socket — used throughout for housing bolts, bracket bolts, and the throttle linkage nut
- Long needle-nose pliers — recommended specifically for pulling the thermostat housing bolts so you don’t drop one down into the engine
- Zip ties — needed to re-secure hoses and wires after you cut the old ones to move things out of the way
Step-by-Step Guide
Getting Access
- Remove the three 10mm bolts holding the top cover on.
- Push down the plastic tabs on the zip ties and cut/disconnect them so you can move the hoses and wires around.
- Remove the two 10mm bolts holding the plastic piece that bundles the wires together.
Port Side Thermostat
- Disconnect the IAT (intake air temp) sensor and the MAP sensor.
- Remove the breather hose.
- Cut the zip tie and disconnect the trim sensor.
- Remove the three bolts and the metal bracket — I used pliers to get these bolts out.
- Remove the black hose. With all of that disconnected, lay the whole assembly to the side, which gets you into the thermostat housing.
- Remove the bracket on the thermostat housing and the nut on the throttle linkage so you can get to the housing bolt. You’ll need to pull the hose out of its black plastic holder to give yourself room to move.
- With the nut off and the hose clear, move the assembly out of the way.
- Take out the two 10mm bolts holding the thermostat housing on — use long needle-nose pliers here so you don’t drop a bolt.
- Pry the old thermostat out and set the new one in.
- Bolt the housing back down and reassemble everything, except leave the gray hose on top and the plastic wire holder off for now.
Starboard Side Thermostat
- Disconnect the TPS (throttle position sensor) and move the gray hose off to the side. Don’t touch the TPS settings — you don’t need to mess with calibration to do this job.
- Pull the wires and hoses out of the black holder piece and move it out of the way.
- Remove the screw holding the hose in place.
- With the bracket off, move the hose aside and use a 10mm wrench to get the housing bolt out.
- Pull the housing out, remove the old thermostat, and clean out the inside of the housing before installing the new one.
- Reassemble everything.
Pro Tips
- Don’t skip disconnecting anything. There’s a lot of stuff in the way on both sides, and trying to shortcut past a hose or bracket just slows you down and risks breaking something. Take it all the way apart the way it’s laid out here.
- Use needle-nose pliers on the housing bolts. These are exactly the kind of bolt that falls into a spot you can’t get your hand into. Long needle-nose pliers save you from fishing a bolt out of the engine compartment.
- You can service the starboard side without touching TPS calibration. That’s one less thing to worry about resetting when you put it back together.
Final Verdict
This is doable if you’re mechanically inclined and patient about disconnecting things in order — nothing about the actual thermostat swap is hard, it’s just tedious getting to it. Where I’d be honest with you: if you’re not comfortable working around sensors like the MAP, IAT, TPS, and trim sensor without second-guessing what you unplugged, this is a job where a wrong reconnect could cause a driveability issue that’s a headache to diagnose. If you’re confident with a wrench and methodical about keeping track of what came off what, it’s worth doing yourself and will save you a service bill for what’s ultimately a straightforward part swap.
FAQ
How do I know if my Yamaha outboard’s thermostat needs replacing? It’s typically done as part of a 100-hour service interval as preventative maintenance rather than waiting for a failure symptom like overheating.
Do I need to remove the TPS sensor to change the starboard thermostat on an F225? No — you disconnect it and move it out of the way, but the job doesn’t require touching or resetting the TPS settings.
What tools do I need to replace the thermostats on a Yamaha F225? A 10mm wrench/socket for the housing and bracket bolts, and long needle-nose pliers to remove the thermostat housing bolts without dropping them.
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