# Running an outboard without a thermostat



## Coastal Whaler (Dec 28, 2005)

I have a Mercury 2004 40HP 4-STROKE EFI. Last October the temp. alarm started sounding at running speed. As soon as I pulled it back to idle it quit sounding. I took it in and replaced the thermostat. Ten months later, last week it started again. I always flush my engine for at least 5-10 minutes after each use. I just pulled the thermostat and there were salt deposits on and around the thermostat. I put the cover back on without the thermostat and want to run it this way for awhile. Any problems with doing this? I doubt you can tear a motor up running it to cool, but I know you can with it running to hot. Any mechanics off the record here for their thoughts, thanks!


----------



## Muddskipper (Dec 29, 2004)

I ran my Johnson 88 spcl that way for years....all year long...no issues


----------



## Leemo (Nov 16, 2006)

The thermst. is there for a reason, running a motor to cool will allow for carbon buildup, blo-by, screwed up valves etc.. the correct operating temp allows for all the fuel to be "spent" corrrectly, put the thermst. back in, maybe you are buying an aftermarket thermst.?? you'll really screw up a diesel running em' to cool, that motor of yours ain't a Briggs and Stratton, please run it the right way..


----------



## cclayton01 (Jun 6, 2005)

My old '86 Johnson 40hp would do the same thing as your Merc. When I took the motor in to get the thermostat replaced they put in a ss washer in its place. Ran it like that up until I got rid of the motor two months ago, not a single problem.


----------



## Profish00 (May 21, 2004)

Leemo said:


> The thermst. is there for a reason, running a motor to cool will allow for carbon buildup, blo-by, screwed up valves etc.. the correct operating temp allows for all the fuel to be "spent" corrrectly, put the thermst. back in, maybe you are buying an aftermarket thermst.?? you'll really screw up a diesel running em' to cool, that motor of yours ain't a Briggs and Stratton, please run it the right way..


Good answer, could warp the heads as well.


----------



## Leemo (Nov 16, 2006)

ccclayton- the washer just restricted the water flow, thus allowing the engine to run hotter than a straight flow, you got lucky.


----------



## cclayton01 (Jun 6, 2005)

Leemo said:


> ccclayton- the washer just restricted the water flow, thus allowing the engine to run hotter than a straight flow, you got lucky.


I suggested that they put a thermostat back in, but the repair tech assured me the washer would serve just fine. That may be part of the reasoning why that particular dealer/repair shop is closed down now. If I had to do it over again on my new motor, I would want the same part in there.


----------



## sqwaby (Aug 4, 2005)

The days of running with no thermostats are over if you run a comp. controlled engine and you what it to operate at max performance level. Those engines need to be run at specific temp ranges so the comp has a base line to set and adjust fuel/air mix, timing,etc


----------



## Hooked (Oct 15, 2004)

If the alarm stopped when you throttled back I don't think it was an overheat condition. Once it overheats it'll take 15-20 minutes with the engine off to cool down enough for the alarm not to sound -- generally. 
I don't know how the Merc works but on my older Johnson the constant alarm can also be lack of fuel flow. If yours is the same you may have a restriction in the fuel line, clogged filter, etc.


----------



## FormerHR (May 21, 2004)

Leemo said:


> ccclayton- the washer just restricted the water flow, thus allowing the engine to run hotter than a straight flow, you got lucky.


I had a 98ish Suzuki that I ran like this for four years and then sold to a buddy. It's still running fine with no problems. I don't think Clayton got lucky, I know several people that run their older outboards this way.

I also agree with sqwaby about the newer motors, they "think" too much to be able to run 'em like that.


----------



## spotsndots (May 20, 2005)

Go get you some Saltaway which hooks up to your garden hose and flushes the system of any salt and prevents buildup. I know Bass Pro in Katy sells it.


----------



## Coastal Whaler (Dec 28, 2005)

spotsndots said:


> Go get you some Saltaway which hooks up to your garden hose and flushes the system of any salt and prevents buildup. I know Bass Pro in Katy sells it.


I have just started using saltaway. I have almost used up the small container that the attachment comes with. I ran it this morning without the thermostat. It ran really good. I was able to get 6000 RPM instead of 5500 that it has been getting in this hot weather with no alarms. Smooth at idle and at all RPM's. The small exhaust hole(not the pee hole) was still really hot to the touch, but not burning hot as it has been. I had the shop replace the thermostat and water pump last October. When I pulled it yesterday there was considerable corrosion around the thermostat. I cleaned the entire inside area with a stiff toothbrush and diluted saltaway. I even noticed some pitting on top of where the thermostat sits. I was not very happy with that on a 3 year old motor. My guess is last year when the shop changed the thermostat out they did not clean the area around the thermostat, just put a new one in. Maybe they did and the corrosion just built up that quick in 10 months. Like I said I always flush with the ears at the water intake, just as the shop suggested. I am leaning toward running it without a thermostat until the water cools below 70 and then put the thermostat back in for the winter.


----------



## NSF15 (Aug 27, 2007)

I wouldn't run my motor without it. As others have sais it is there for a reason. A good friend of mine is the service manager for a marina and whn I have asked him he has growled at me never to run my motor without it. (Not sure what he meant by the growl.)


----------

