B90TLRW - 90HP 2 Stroke Thermo question

[Fish Nut]

My frist post here so please be gentle .

I've been paying the mechanic all these years but I finally got up the courage to change the thermostat and water pump in my F6 kicker. It turned out to be a lot easier than I expected and actually somewhat rewarding so now I'm going to do the main engine too which is a 1989, 90HP two stroke.

I ordered the new thermostat and water pump kit but as I did some more research I see in addition to the Thermostat (pn 668-12311=11) there's also a "Thermo Switch" - (pn 668-82560-10-00), should I replace that too while I'm in there or wait until I have a problem? As far as I can tell there is only one thermostat but I hear a lot of engines have two of them, does anyone know if this engine has one or two thermostats and where the second one would be?

I'm not having any issues with the engine but after I saw how corroded and salty the F6 was I figure this one is probably worse.

[robert graham]

Just one thermostat on that motor, but maybe also replace that Pressure Control Valve that's right under the thermostat, while you're in there. I wouldn't replace that Thermo-switch as long as it's working O.K. Mine is a 1999 90C and these motors seem a lot simpler and easier to work on, especially after you dive in there and do it once. But it's only natural to be a bit scared of something you ain't tackled before!

[Fish Nut]

Thank you! I'm looking at the parts diagrams listed at 1998 B90TLRW Yamaha Outboard Parts but I don't see the pressure control valve listed but I may be looking in the wrong section.

Hopefuly not related to the Svetzer valve if you know what that is...

[robert graham]

My motor is a 1999 90C model, so may be some different from yours. The Pressure Control Valve on mine is a black plastic item with a coil spring and it sits under the same cover with the thermostat...maybe your motor doesn't have one?

[boats.net]

Pressure control is number 41 in this diagram, 1998 B90TLRW Yamaha Outboard CYLINDER CRANKCASE Diagram and Parts

[robert graham]

Pressure control is number 41 in this diagram, 1998 B90TLRW Yamaha Outboard CYLINDER CRANKCASE Diagram and Parts

Yep, there it is under, Cylinder Case Diagram...might also want to replace that item #40(rubber grommet)....you'll need a new gasket for that cover also.

[Fish Nut]

Thanks again, I bought the new gasket when I bought the thermostat. The boat is in my garage so I think I'll pop out the old thermostat tonight and inspect those other parts before I order new ones. Again, this is the first time I'll be in there on my own and not sure if my mechanic ever replaced the Pressure Control Valve so I want to see what kind of shape it's in before I order a new one. I don't remember the mechanic ever mentioning that was part of the regular maintenance but could easily have forgotten.

One more question (probably not...) Should I use liquid gasket sealer (adhesive) on the thermostat gasket or should I put the new gasket's in dry with no sealant? It seems like the one in my 6HP 4 stroke was only sealed on the engine side, meaning, the thermostat cover came right off but the gasket was stuck to the engine block and looked in good shape. I assume that side was not sealed to make it easier to repair next time. I didn't notice any salt build up on the engine block and I doubt there's much pressure in there, just wondering what you all do?

[robert graham]

I just used marine grease on both sides of that gasket...so hopefully if I have to remove that cover again the gasket won't stick to the surfaces. I just replaced that Pressure Control Valve, Spring, and Rubber Grommet, but after inspection and a good cleaning, I believe the 3 parts were still good and usable, so I'm keeping them for spares...might save you some money! I cleaned and saved my old thermostat also. This was the first time since motor was new(1999) that these items had been replaced and there was a good bit of calcium/mineral/corrosion deposits in and around them...although the components themselves appeared to still be O.K., especially after cleaning! Good Luck!