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