F350 Dually with no pick up

I have a 2001 F350 7.3L Dually and I have been very happy with it. However, on cold (below freezing) mornings when I don’t plug it in, the engine will start fine but until it warms up it will only go about 25 mph. It also makes a swishing sound below the passenger seat. It feels like it is being held back. Any help you can give me I would appreciate.

There is a butterfly valve in the turbo outlet that is closed under certain conditions. It’s main purpose is give resistance to the enginge so that it warms up faster than w/o it. There are some DOT and EPA requirements for the engine to get to temperature within a certain time. Anyhow, that valve is there to “load” the engine. There may be some calibrations that use it as a brake assist (i.e. exhaust brake). The valve will open if you push the throttle down far enough.

Thank you very much! Can it get frozen and not respond in the cold weather? I have the throttle to the floor and I only get 25mph and I get there very slowly. If I don’t have it in gear the engine will throttle up normally.

It is computer controlled via an oil solenoid. You can see it at the back of the turbo…a ~1.5" cylinder with some linkage. I’ve not observed any stuck ones but that doesn’t preclude it happening.