Cummins 12 valve 1400° OUCH help!
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I'll leave it to some of the other guys here for confirmation, but it seems like 1200* is the ceiling for safety. Assuming the turbo has consistent boost behavior from when the truck has just been started and is cold through when it has warmed up I think you'll find a wastegate signal line issue. I'm not sure what year your truck is but it'll have either a WH1C or an HX35 and both of these are simple to diagnose. Apply ~30 psi of air to the wastegate and see how much travel you get. You should get 0.5 to 1.25 mm travel from the wastegate actuator rod. If it's in spec start checking the line that supplies the actuator.
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