OMC 40/48/50 Thermostat problem

[Oldslowandugly]

[On an OMC 40, 48, or 50-HP outboard engine,] when you change the thermostat the instructions say to change the seat at the same time. The seat is a rubber ring that the thermostat sits in. I always do, but the last time the thermostat suddenly ran cold. I suffered through the end of the season with a lukewarm motor.

Today, the thermostat worked fine when I checked it in a pot of boiling water. That seemed odd. I went to put it back in the motor, and, when I did, the seat fell out. Usually they are stuck in pretty good and have to be pried out. I noticed, as I worked, [the seat] kept falling out--that's when it hit me. The seat is designed to wedge into its orifice and stay in place.

At high engine RPM, the water pressure pushes the spring loaded thermostat off the seat, the water bypasses the thermostat, and the motor runs cool. As the motor slows down the thermostat re-seats and warms up the powerhead up to operating temperature.

For whatever reason, this seat would not stay put. When the water pressure unseated the thermostat, the seat came loose and jammed the thermostat in an unseated position. I had a new thermostat kit handy, so I pirated another new seat and used it with the old thermostat. It works fine. Now I know to stock up on seats and change them before I waste money on an expensive new thermostat.

[OldKenT]

Hello Oldslowandugly,
Have you found a difference between OEM and aftermarket seats and thermostats?

[Oldslowandugly]

Yes, I have wasted a lot of money on Sierra products and I have been disappointed plenty of times. Sierra electrical items have been particularly bad. Sadly, many items are only available through them and I don't understand why they fail since Teleflex owns them. I now go out of my way to stick with genuine OMC or Bombadier products even if it's a 40 year old NOS part on Ebay.

[Oldslowandugly]

I should add something else I found out. The stock spring is designed to let the water unseat the thermostat at a predetermined pressure that is in concert with the motor rpm. I always felt it unseated early and cooled down the motor too soon . I took some small stainless steel washers, the same diameter as the spring, and placed them behind the spring in the cover. That caused an increase in the spring pressure which in turn increases the rpm at which the thermostat unseats. Each washer I added raised the event by about 500 rpm and too many caused it to never unseat. I settled on a single washer which set the event at 2800 rpm. Above that speed the boat starts to plane so that works out perfect for my situation. Now I can run back for another drift without cooling the motor down too much yet when I need to travel on plane it runs cool.

[OldKenT]

Thanks for the information. Just a thought, if the spring has weakened over time and lost some of its tension, perhaps it needs to be replaced. I have had my 1989 Johnson 40hp for only two years, but so far have not had a cooling problem.

[Oldslowandugly]

I bought a new spring thinking the same thing. It was fine. I have three now and they all are as strong as day one. I even tried over stretching the spring to cause more pressure but it would not stretch at all. It is a very strong grade of stainless steel and very durable. I had my 1993 48SPL for 10 years before the thermostat acted up. That is the one I changed out and only a month later it started running cool. I blamed the thermostat when it was the seat all along. I have had plenty of motors and always changed the whole assembly. This is a first.

[Oldslowandugly]

I launched the boat yesterday and all was well. Then when I arrived at the marina, I noticed by the temp gauge it was running cold again. Back to square one.

[Oldslowandugly]

I went back today for a look and sure enough, the seat was out of place. I don't get it.

[Oldslowandugly]

So I tried another tactic that seems to work. I reset the seat and thermostat. Then I ran around for a couple of days never going over 2000 rpm. That way the thermostat alone controlled the temperature and it never unseated. The motor ran at 140-160 degrees where it should be. Then on the third day I let it get good and warm, then revved up to 4000 rpm and let the thermostat unseat and the motor cooled off as normal. Then I slowed down and watched the gauge. Bingo! The seat stayed in place, and the motor warmed right back up. So I guess letting the motor run warm for those days caused the seat to conform to its cavity, and it looks to be working as it should. Keeping fingers crossed.

[Oldslowandugly]

UPDATE.

All summer the thermostat seat kept coming loose. I didn't mind because once the water was warm it didn't affect anything. Now the water is cooling and it matters. So I tried another trick.

I removed the seat, and cleaned it and the opening it sits in with acetone. Then I coated the seat and the contact area with good old OMC Adhesive M. I pressed it into place with a wooden dowel and then let it air cure for an hour or so. Make sure no adhesive gets on the part the thermostat touches or it will not unseat when it needs to.

Then I cruised around slowly so that the motor got warm but not fast enough for the thermostat to unseat. The Adhesive M is cured by solvent evaporation as well as heat. By getting the motor warm I was hoping to accelerate the cure. That seemed to work.

Later as I was cruising I spotted Bluefish breaking and I instinctively shot after them. The thermostat unseated as it should. When I slowed down it re-seated properly and the motor warmed up again. The rest of the day it seemed to be working well but only time will tell for sure if the problem is finally solved.