More boost = more backpressure. Backpressure hurts effiency, buuuuuutt I had read a post on another site that hinted that higher egt's AND boost while cruising MAY increase mileage. I have yet to understand the theory behind this, just throwing it out there. The common thought is that the lower the boost AND egt's the better the economy.
The wastegate is not open at cruising speeds unless there is an issue with the actuator of the gate itself. 4 psi at cruise sounds VERY normal but it should jump higher than 12 when you mash on it.
Is is downshifting out of OD, or do you have a manual?
Have you checked for any pre-turbo exhaust leaks, or any boost leaks?
There are a couple ways to increase boost i.e boost fooler, aftermarket actuators, or just by simply disconnecting the red tube that goes to the actuator. Beware that these all may shorten the life of your already over taxed turbo on a chipped truck, and more boost does NOT always mean more power.
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