This thread is perfect timing.
I'm getting down to the nitty gritty on choosing which truck to buy, and warm up is one of the things I'm looking in to.
It's between a 21 Cummins and a L5P. I just can't bring myself to trust a Powerstroke as long as it has the CP4, no matter how much they claim it's not a wide spread issue or concern.
From what I've read the Cummins block heater works whenever it is plugged in and goes to about 100 degrees. I know the Duramax block heater doesn't even come on until it's below 0, so warmup from cold is important.