mix the ratio of coolant to water with more water in the mix.
water is able to carry heat away better than coolant.
50%water50%coolant is 265 boil F over protection. if you drain off
some of the mix from your system and add just water that would thin the mix
of coolant and give you a higher ratio of water to coolant. There must be
somewhere on the
internet that tells us what the boil over protection is for every ratio of
coolant mix. Basically
for a boat that runs 190 like yours, this would lower the heat of your
running engines some,
maybe back to the 160-180 range.
I don't know if you have a radiator on your cooling system, but on an old
radiator if the foil is
rusted away from the tubes, then the radiator itself will not be able to
draw away as much heat from the cooling system. Sometimes by painting the
radiator, I use heat resistant paint like the kind you
paint car headers with, it will help cool the engine some.
http://users.rcn.com/neptunium/cool/cool.jpg

hope this helps
greg
"Tom "HappyFunBoater" Treadway" wrote in message
m...
I've got two Cummins 420hp diesel engines. They're both about 5 years old
and I've never changed the coolant, but I have made sure it was topped off
through the years. So two weeks ago I decided to replace the coolant.
When
I drained the old coolant, everything looked fine, i.e., no rust or
particles. Then I refilled, slowly, with a 50/50 mix with Prestone
coolant
that met the engine specs. I didn't start the engines until this
weekend -
two weeks later.

Before I changed the coolant both engines very consistently ran around
170F.
The engines are rated for 160-180F. Now both engines run around 190F -
20F
hotter! I ran for about a hour thinking that maybe the coolant needed to
mix, or air bubbles needed get out of the system, but it made no
difference.
The temperature is dead on 190F.

What the heck did I do wrong?

Thanks for your help.
TT