Mercury 115 2-stroke thermostat

[ticondo46]

I have a 1997 Mercury 115 (four cylinder two stroke) outboard that runs fine. I've never changed the thermostat on it because it seems to work fine (I have an engine temperature gauge so I can monitor this closely).

However, there seems to be a recommendation in these forums to change the 'stat every two or three years. The reason I never did it until now is that on this particular engine, the thermostat housing is held in place by several cylinder head bolts. I personally am unhappy to unscrew three or four cylinder head bolts without slacking off all the other bolts. But if I do this, I'm pretty sure that I am going to run into head gasket problems, which means that I may well have to change the head gasket at the same time as the thermostat.

Has anybody attempted (successfully) to unscrew only the relevant head bolts, with no damage to the head gasket?

Thanks!

 

[ENSIGN]

Re: Mercury 115 2-stroke thermostat

If there's not a overheating issue,leave it alone...

 

[Texasmark]

Re: Mercury 115 2-stroke thermostat

"Don't fix it if it ain't busted". Many things affect the life of a stat. You have a temp monitor and also an overtemp alarm that shoots off at about 196F, stat starts opening at 143 and should be fully open by 153, is only used at low speeds.....above about 2500 rpm it is paralleled with a much larger opening called a popoff valve which is pressure controlled. If your stat were to stick shut, which I had happen to my engine on my first outing on my used OB, once you get up on plane after a few seconds the alarm shuts off and stays off till you get back to the dock and slow down.

That engine, like mine and most Mercs has no head per se...head is part of the block. To check for yourself, look at the side of the plate you call the head. You will only see one gasket.....all you have is a water jacket cover. If you had a separate head, you would have 2 metal plates and 2 gaskets, the second being the head and head gasket.

I have the little brother to yours and I just looked in the service manual for the reference to screws you mentioned. These screws just seal the water jacket and if you take them out and only prise off the cap over the TS and popoff (the lower and larger diameter of the cover), you can put her back together and not disturb your water jacket seal. I replaced my stat back in '05 and didn't even put a new gasket on it.....didn't have one. I just took some blue RTV and glued the portions of the orig stat gasket I tore upon removal and smeared some on the entire flange, bolted her back together and have had nothing more to do with it.

Post edit: Yeah I did go back there on one occasion later on. I took some Phantom Black paint and painted over the blue RTV that oozed out from putting things back together so she would still look new. Ha!

HTH,

Mark

 

[ticondo46]

Re: Mercury 115 2-stroke thermostat

Thanks, Texasman and Ensign, those are great and clear explanations. So basically, I shall do nothing and simply keep on enjoying my outboard without useless preventative maintenance (BTW, I must admit that never changed the pump impeller; the engine has more than a 1,000 hours on it, and I've checked the impeller every now and then, but it still looks new so why change it? About the cylinder head, your observation is absolutely correct. It doesn't show clearly on the Mercury Maintenance Manual, but I always wondered why they only referred to a "cylinder cover" instead of a "cylinder head" (and their illustrations are less than clear).

iboat forums are really top quality, thanks for your help!

"Don't fix it if it ain't busted". Many things affect the life of a stat. You have a temp monitor and also an overtemp alarm that shoots off at about 196F, stat starts opening at 143 and should be fully open by 153, is only used at low speeds.....above about 2500 rpm it is paralleled with a much larger opening called a popoff valve which is pressure controlled. If your stat were to stick shut, which I had happen to my engine on my first outing on my used OB, once you get up on plane after a few seconds the alarm shuts off and stays off till you get back to the dock and slow down.

That engine, like mine and most Mercs has no head per se...head is part of the block. To check for yourself, look at the side of the plate you call the head. You will only see one gasket.....all you have is a water jacket cover. If you had a separate head, you would have 2 metal plates and 2 gaskets, the second being the head and head gasket.

I have the little brother to yours and I just looked in the service manual for the reference to screws you mentioned. These screws just seal the water jacket and if you take them out and only prise off the cap over the TS and popoff (the lower and larger diameter of the cover), you can put her back together and not disturb your water jacket seal. I replaced my stat back in '05 and didn't even put a new gasket on it.....didn't have one. I just took some blue RTV and glued the portions of the orig stat gasket I tore upon removal and smeared some on the entire flange, bolted her back together and have had nothing more to do with it.

Post edit: Yeah I did go back there on one occasion later on. I took some Phantom Black paint and painted over the blue RTV that oozed out from putting things back together so she would still look new. Ha!

HTH,

Mark