So I will try to explain this the best that I can and hope that it makes sense. I have talked to the local mack dealer, and pretty much everyone that I know and I haven't found an answer yet. First off my 2008 MP8 will build and hold 42 psi of boost on a good long pull. It sits at idle with around 3 psi. Sometimes in between gears or if i gave to stop really fast (like throw it in neutral and slam the brakes) my truck will be at idle with 12- 13 psi of boost. the real weird one is this, lets say I'm driving the truck down the interstate at 65 mph. the road is either flat or has a very slight downhill grade with the engine brake turned off, cruise control turned off, not on the gas brake or clutch, just free coasting down the road, the truck build 12 psi boost and start to slow the engine down. The effect is not as strong as the engine brake on the low setting but it makes a similar although quieter sound and does very slightly try to hold back the motor. The actual speed doesn't matter, the grade doesn't matter, and the gear doesn't matter. I did replace to boost pressure sensor. This problem has existed for a while but now and then more recently during a driving regen the turbo will spool up so hard the the boost gauge goes to max and something rather load pops under the hood. any thoughts or idea would be great. I really don't want to start out by replacing the turbo, but I will if it comes to that.
-Thanks
Spring