Have you blown an CAC boot? I would think if the veins were sticking due to rust/soot, it would overboost, not underboost. If the veins are stuck, they would be stuck in "small turbo" mode and it would spool up in a heartbeat and blow a boot, or possibly a tank off the cooler itself. I am thinking a sensor problem. What boost gauge are you reading? A stock, or "tuner" gauge? Or is it a real gauge like in your brown truck? When the CEL light is on and the boost gauge is showing 0 PSI, do you still hear the whistle? A 6.0 whistles like nothing else I heard..... I love how mine sounds.....
One thing is, there is some truth to the run it hard the D-Max guy was telling you. With the EGR and the VGT, a 6.0 dose not like to be babied. Do not thrash it, but when up to normal operating temp, at least once or twice a week you should give it a couple good heavy throttle take offs..... Not to the floor from a dead stop, but once you are rolling, roll into the throttle and go to WOT, like getting on the interstate..... I have driven mine like this since I got it, and have the factory turbo and EGR in it. They have not been cleaned.... ever. Also, do not let it idle excessively..... they do not like that either. If it does idle for over 5-10 minutes, the next couple of take-offs, get on it and blow any soot out while it is still "fresh" and has not set up.....
I would check the boost with a real gauge if this is a factory gauge... and go from there....
Also, what are yes seeing for boost numbers when it is running right, and no CEL on? I know that a 6.0 makes boost almost all the time. If you are seeing numbers like you would expect from your brown truck, all the time, something is wrong. 10-15 psi is low to middle numbers for a 6.0. On a WOT loaded acceleration you should see almost 30 psi of boost. If I remember correctly, an overboost CEL is set at something like 30 or 32 psi on a stock truck. My 03 does not have a boost/pyro yet, and I am running 100% stock and I have not set any over/under CEL's but if it was indeed not boosting, you would know it.... I would think it would either smoke like a coal truck, or de-fuel to the point it would not pull itself.
What are the specs on this truck? Auto or 6 speed, Axle ratio? Miles?
On mine, an 03, 5 speed Auto, 3.73 axles... you can FEEL the boost. If I do a WOT take-off, like I described above, it keeps the boost/fuel lower through 1st and 2nd. When it hits 3rd, it will really fuel, not rolling any coal, it may haze a bit, but you can feel the boost come up, it will set you back in the seat and just flat go until you let off the throttle. It will run like that all the was up to the speed limiter, not the governor. A 6.0 PCM/FICM will de-fuel at ~98 MPH. It will keep you in the seat until there, or you let off...
