I believe the thermostat in my Mercury 115hp 4-stroke outboard is
broken. Now, I need info about which thermostat that I should use to
replace it.

The existing thermostat didn't pass the hot-water test. I heated the
water up to 180 degree, and it didn't open in any way -- not even
slightly. I believe this means the thermostat is dead.

But I have a question as of which temperature range of thermostat that
I should get. The Mercury Service Manual doesn't say which temperature
range to choose; it simply tells me to check the temperature rating
stamped on the bottom of the termostat. When I look at the bottom of
the old termostat, it has a "120" marking. I guess this means the old
termostat is supposed to start opening up at around 120 degree; and I
also assume that this means the old termostat is supposed to maintain
the water temperature to no less than 120 degree, right? According to
some past posts, 120 degree is kind of low temperature and is supposed
to be for people who like to run the motor at slow speed -- as in
trolling. Do I have to stick with 120 degree termostat like what the
Mercury Service Manual has suggested? Should I replace it with a 143
degree termostat that I am under the impression is the more common
termostat being used in Mercury outboard (if I read the SELOC Repair
Manual correctly)? You see I am planning to add a 9.9hp outboard to
the boat for trolling, and will likely use the main outboard for
cruising; this means I will not run the main outboard in slow speed all
the time. Does this mean that I am better off using a 143 degree
termostat? I will use the boat in both fresh water (25%) and salt
water (75%); I mention that just in case this makes a difference.

Based on the equipments left in the boat, the ex-owner of the boat
didn't fish much (no fishfinder, no live well, only find one rusted
hook left). Then why would he put a 120-degree thermostat in the motor
that is for people who like trolling? Is there any other reason why a
boat owner or the boat manufacturer may want to put a 120 degree
thermostat in the motor?

I have a couple questions related to the old/dead thermostat:

1. What does "sticking termostat" mean, like when people say "the
sticking termostat is causing problem"? Does this mean the termostat
sticks at the "close" position and cannot open up like what my old
thermostat is?

2. What's the effect of having a thermostat that refuses to open? I
am asking this because I have a problem getting water coming out from
the telltale, and I have a hard time finding out the cause of this
problem, and I am wondering if a constantly-closed thermostat may cause
this problem or not.

Thanks in advance for any info.

Jay Chan