Re: 82' Merc 40 HP ok to run without t-stat ?
i have a 1982 Mercury 40 hp. 2 cyl. i would like to remove the thermostat permanently but i dont know if that will harm something, over heat etc., is the purpose of the t-stat to reduce water flow so the motor runs warmer in cold conditions or is it to increase water pressure at low rpms for cooling efficiency?
Without the t-stat would it run too cold in cold water/conditions or would it overheat due to insufficient water pressure ?
Or would it really not make any difference?
This topic has appeared many times in this forum and I believe I have read them all and responded in some cases. My interest in the 33cu. in. 35 HP Mercs began when I purchased a 1987 (0B177575) model 5 years ago. Previous to this, I had 40 years of experience with OMC products. Quickly I discovered this 35 merc had no thermostat. My gut feeling and common sense told me that all outboard engines should have a stat for max operating efficiency.
Looking through parts breakdowns, I have discovered this:
Looking back from 1983, these 33 cu. in. blocks were rated at 40 HP, then from 1984 to 1989, rating was 35 HP, crankshaft HP rating vs prop shaft rating, no change really.
But, the cooling method did change, from thermostat and poppet valve in 1983 to none of these parts in 1984.
Also the route for the cooling water was redirected. In 1983 and below, water was directed from the water pump to the engine block and into the exhaust plate area at the bottom of the engine for cooling exhaust and also into the cylinder block area. A parallel path at this point was also provided to direct water to the fuel pump (for cooling???) and then out to the pee hole. This meant you had a pee stream immediately with no indication if the thermostat was open or closed. If it was stuck closed and the poppet failed also you had no indication of overheat since these engines then had no warning horn since they had no overtemp sensor in the block.
Now on to 1984. The water from the water pump still arrived at the base of the block, but the previous path to the fuel pump was eliminated and the hole in the block was plugged. Water was then allowed to flow freely into the exhaust plate area and into the cylinder block area and into the housing that previously held the thermostat and related parts. Fuel pump design was changed also. The cover for the thermostat which had a hole in and was now replaced with a solid cover and o ring seal. An elbow connection was added to the thermostat houising for a hose connection to the tell tale, this method takes a few seconds for the tell tale to indicate. You then knew you had cooling water flowing if the tell tale was flowing since the engine block was full of water to the top. A much simpler and reliable method. Incidentally, this was also the year when a warning horn and overtemp sensor was added.
As for max operating efficiency on a no thermostat engine? I trust Merc has worked this out since my 1989 35 HP runs just fine in the cool waters in Maine in early spring for salmon fishing. Fuel economy is very good.
I recently bought a 1982 Merc 40 HP and noticed an intermittent tell tale. When I took apart the water pump, I noticed the impeller had disintegrated. I correctly diagnosed that bits and pieces were somewhere in the cooling path. I found the pieces and while all the plates were off, I studied the cooling paths for this engine and compared it to a carcass of a 1987 engine that I had. Looks the same to me. The 1982 works fine now with it's conversion to the post 1983 cooling scheme .
So, you choose which way to go
John
