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As stated in the subject line I want to add a temp gauge on my '96 Merc 25hp 2 cycle outboard. I find no obvious plug on the cylinder head to put a sending unit (only a aluminum plug disk w/numbers on the port side of head which I assume is a freeze plug??)
My question how do I add a temp gauge on this motor. (S/N 0G034042) I know there must be some Merc experts on this forum that can help a fellow Whaler.

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Why? I would spend the money on a new water pump and make sure its working, this is more important than a temp gauge. Also more important would be to have a water pressure gauge this tells you the water pump is working... Do not take out the freeze plug either...

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There are 2 types of temp gauges that I know of...

1. Cylinder Head temp
2. Water temp

It sounds like you are looking for a head temp gauge?
Not sure if smaller motors have this fitting outlet(s).....

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Thanks Joe and Contender.

Contender, I'm not familiar with the water pressure gauge. How is this installed? I appreciate your comment concerning the water pump, but pumps can fail after installation, inlets can get clogged and so forth, that is why I want a gauge to let me know how the cooling system is working, be that a water pressure gauge or temp gauge. BTW, I had no intention on taking the freeze plug out, I was just stating that was all that I could see.

Joe, you mentioned two types of gauges, what's the difference between cylinder head temp and water temp? If there is no fitting (outlet) for a head temp, where would I access the water temp with a sensor?

I spoke with a "shade-tree mechanic" (of course most of us are) mentioned "drilling and tapping" for a sending unit. That sent cold chills down my back, I could not imagine taking that route.

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I want to add a head temp gauge to my outboard as well. Not only does it tell you if your water pump is working, but assures you your termostat is working, tells you your water jacket is clear, and warns you of any freak problems that could lead to an overheat. About your engine, I would try to contact your local merc dealer.

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Water pressure gauge works off the tee, were the water comes out of the engine after it has been pumped, (instructions will show you this, hardest part will be running the hose and installing the gauge) If the gauge is not working either you have a blockage or your pump has failed. another issue the forward motion of the boat forces water up the pick up into the engine, as far as a blockage, if your screens are in place and you flush the engine after each use this should never ever be a problem. I have never heard of a water pump failing after being brand new and installed (anything can happen but does not sound logical)

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http://marineoutboardmonitor.com/nitor.com/

Here is information on a cylinder head temperature gauge system that I have been developing. I place a thermocouple under one cylinder head bolt for each cylinder, use one digital display and a couple toggles so I can read the temperature of each cylinder.

For those seeking a simpler single water temperature system a standard 12 volt digital dash style meter is available from 25 to 50 dollars with a temp sending unit that would require a tapped hole in the water jacket. For another 20 dollars a temp sending unit that fits into existing water hose is also commonly available.

typical 12 volt meter and water jacket sensor

http://www.ebay.com/itm/300545412875?...1423.l2649

A sensor for insertion in water hose which is also available in other common sizes.

http://www.ebay.com/itm/190603713508?...1423.l2649