How does desired boost work?

[pistonrings]

I've replaced my stock turbo with a fixed vane type. I was just curious what the ECM tries to do when the actual boost pressure exceeds the desired boost pressure. In my case simply moving the vanes is obviously not an option. When towing a trailer, it seems like the truck is de-fueling to try to keep the boost at a reasonable level. I was just wondering what exactly is happening.

[LBZoom]

In the case of fixed vane turbos, the engine has to de-fuel to lower the boost pressure if simply opening the wastegate isn't enough. Mechanical turbos rely on wastegate and exhaust gases to achieve a desired amount of boost, in order for the ECM to reduce boost either a.) the wastegate opens or b.) the engine has to de-fuel to reduce the amount of exhaust gasses which are responsible for spooling the turbo, which is what you truck seems to be doing from what you describe. The way to prevent this if I'm correct would be to increase the max allowed boost in the ECM boost tables, if that doesn't work increase your desired boost tables as well. Also, I was confused when I first started but the desired boost values require you to subract barometric pressure to get the actual pressure that you'd see on your gauge. (i.e. if the desired boost value says 20 and the barometric pressure is 14, your actual boost is 6)

[cumminsDK]

if on an lly the truck won't run very good if it sees more than 5.1 psi difference between actual and desired boost levels. but this can be turned off in engine diagnostics by raising the low rpm limit for boost monitoring