So I recently picked up a ‘93 K3500 with the 6.5L diesel. This is my first truck so please excuse the newbie questions.
I have made several trips with a steel 3 horse BP trailer (approx 3,000lb curb weight) back and forth along highway 3 in BC Canada. For those that don’t know it’s fairly mountainous, some steep grades, the worst of which is 9% I believe.
Now I know this truck is never going to win any speed competitions, but I expected a tad more power. Going up hills she sucks, and not in a good way. I thought it was an air problem at first but after shaving two hours off my best (empty truck, empty trailer, in Drive) time by being in Overdrive (with no load) I got to thinking.

So I did a test:

In Drive, the truck struggles to maintain speeds over 80km/h and is running about 3100rpm. She has zero power on a hill, can’t maintain speed, and the pyro tells me she’s hot.

In Overdrive, the truck cruises and easily does 120km/h without passing 2500rpm.

Today’s test was with an empty truck.

Is this normal for this truck? Is there something I should have looked at? Again, I know she’s not built for speed but I thought she’d be able to do a bit better ...
I didn’t get a lot of history from the guy I bought it from but he told me the glow plugs and wires were done recently, and he upgraded to a 4” exhaust (but did not do the crossover pipe). I know she needs her injectors done, they’re on order, along with new lines.