I know nothing about Duramax diesels but ...
I was having a problem with my gas 98 Chevy taking longer than normal to warm up and it turned out to be a faulty radiator cap. It was only holding 13 psi instead of the required 16 psi. I would never have believed that a slight difference in pressure would make a difference but it did. Pressure and heat go hand in hand.
My first thought was a bad thermostat so I put in a new one but no change. Then I swapped out the radiator cap and that cured the slow warm-up problem.
