Answer
Mar 26, 2025 - 06:41 AM
Changing the thermostat on a 90HP Mercury Optimax DFI is actually quite straightforward as long as you can get to it easily. The thermostat is located on the starboard (right) side of the cylinder head, inside a small housing that's held on with two bolts.
Start by disconnecting the battery just to be safe, then trim the engine down for better access. You'll want to remove the cowling and side covers to get to the thermostat housing.
From there:
-
Take out the two bolts holding the housing cover.
-
Pop off the cover. Be careful here; you might get a little coolant spill, so have a rag ready.
-
Remove the old thermostat and note how it’s seated (spring side usually goes in toward the block).
-
Inspect the gasket or O-ring and replace it if it looks worn or cracked.
-
Drop in the new thermostat, reseat the gasket or O-ring, and bolt the housing back down snug.
Once it’s all buttoned up, start the motor and check for leaks, and make sure the engine warms up and maintains proper temperature. If it still runs cold or overheats, the manual gives you specs for checking poppet valve function and water pressure too.
In any case, you can find the manufacturer's procedure for thermostat replacement on the manual. If you're doing this DIY-style, the OEM manual would be your most accurate companion!
