Running an outboard without a thermostat - 2CoolFishing
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Old 09-06-2007, 01:27 PM
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Running an outboard without a thermostat

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!
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Old 09-06-2007, 01:31 PM
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I ran my Johnson 88 spcl that way for years....all year long...no issues
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Old 09-06-2007, 01:32 PM
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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..
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Old 09-06-2007, 01:33 PM
cclayton01 cclayton01 is offline
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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.
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Old 09-06-2007, 01:34 PM
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Quote:
Originally Posted by Leemo
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.
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Old 09-06-2007, 01:35 PM
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ccclayton- the washer just restricted the water flow, thus allowing the engine to run hotter than a straight flow, you got lucky.
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Old 09-06-2007, 01:54 PM
cclayton01 cclayton01 is offline
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Quote:
Originally Posted by Leemo
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.
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Old 09-06-2007, 09:15 PM
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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
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Old 09-07-2007, 07:28 AM
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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.
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Old 09-07-2007, 07:52 AM
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Quote:
Originally Posted by Leemo
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.
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