1995 7.4 merc will not heat up

"Need help 1995 7.4 carb model merc. engine will not heat up. Have changed thermostat and checked temp guage but the only time it will go past 100 degrees is when you run for a period of time, shut the engine off briefly and then restart the temp guage will go up to 175 for just a few seconds and then drop back where it will not even move the temp gauge. Sounds like to me it is bypassing the thermostat somehow. Please help"

FWC or raw water system?

FWC or raw water system?

"I do not know what FWC is but

"I do not know what FWC is but it is not a closed cooling system but cools from fresh water from lake, the thermostat housing has 4 hoses coming into it."

"Is #1 below your thermostat h

"Is #1 below your thermostat hsg.? PN 807252T1 is a 140 deg. F. T'stat. Get a laser thermal temp. gun and tatke temp. readings and let us know what you find. Hsg. may need work/replaced.

http://www.mercruiserparts.com/Show_Pictures3.asp?dnbr=82172392&ivar=images/CRUI SER/821723.92/15.png&inbr=1621&bnbr=160&bdesc=THERMOSTAT+HOUSING+%28STANDARD+COO LING%29%28S%2FN%3A+F3519999+%26+BELOW%29"

"This is not what I have close

"This is not what I have close but the housing sits vertical with the motor, serial number on motor is 0F557240, will not be able to check with laser gun until next spring winterized for this season, any other suggestions I am planning on putting a new thermostat in next spring and try that, but need the drawings to make sure that I put in the correct way, plus I do not think there was a sleeve in it when I replaced it last time"

I was thinking those poppet va

I was thinking those poppet valves were stuck open.

"Any other thoughts on why thi

"Any other thoughts on why this boat will not heat up, it use to stay at about 170 at normal operating temp before I changed the thermo a few years ago now it has been running very cool, which I thought was OK but someone told me that if it always runs to cold that is a problem also."

I have tried to send pictures

I have tried to send pictures of the thermo housing but it will not let me. Do you have drawings of my thermostat housing.

"This is the one then. T'

"This is the one then. T'stat is most likely not closing. Make sure you use exactly what you see in the drawing. If the internals of the hsg. are full of rust then have it cleaned at a radiator shop or dip it clean in muriatic acid--DANGER!!!--SEVERE BURN HAZARD!!!

http://www.mercruiserparts.com/Show_Pictures3.asp?dnbr=82172392&ivar=images/CRUI SER/821723.92/16.png&inbr=1621&bnbr=170&bdesc=THERMOSTAT+HOUSING+%28STANDARD+COO LING%29%28S%2FN%3A+F305000+%26+UP%29"

sorry for the late reply. i h

sorry for the late reply. i had the same problem on my 1990 7.4 merc. when i removed the thermostat and housing everything looked good and the thermostat was functional. i was totally confused!! after a couple beers i noticed that there was an impression on the thermostat housing gasket. the thermostat was not seating correctly and raw water had been washing by and not allowing the engine to get up to operating temp. are you certain that the thermostat is fitting snug inside the housing and not off to the side?? you would be surprised if you had seen how small the openeing was that was a result of the thermostat not in the right position and what a huge difference it made in the operating temp. I couldn't get the engine over 100 degrees and as soon as it was installed correctly it was at exactly 145.