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Boost vs. mileage
Maybe it's not the boost but EGR? On a gasser EGR reduces pumping losses
but on a diesel it slows down combustion, reduces peak temperatures (and thus NOx), makes BSFC worse. That is the reason heavy duty trucks in Europe use urea instead (AdBlue). That way truck manufacturers don't have to trade fuel efficiency, particulate emissions, power density, reliability against NOx. Making AdBlue has energy costs too, but those are not taxed so that's what we get.
Back to topic: listen for a hissing sound when the egr gate is opened. I think driving either with just a tad of boost but EGR closed or full throttle (EGR closed again by computer) is best. EGR systems in general are not reliable as you may already know so it is possible you have a faulty EGR valve?
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