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Original post by: Terry ,

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Assuming there are no other symptoms like massive power lag or loss, on the newer ISX as well as the DD15 engines, Boost pressure will quite often spike to max after changing gears to prevent torque lag. After that spike, lasting about 5-6 seconds typically, the ECM then changes modes depending on several factors mainly based on Altitude, outside air temperature, exhaust temp, etc. Basicly, it decides if one of 2 EGR modes needs to activate, or if one of the NON-EGR modes is required. It sounds like in your particular case, it is choosing a NON-EGR mode. ALL NON-EGR modes will in fact drop the boost pressure by about 10-PSI when the EGR becomes inactive. This is to actually maximize fuel economy and torque at the same time by balancing a set-point between providing maximum boost (I am assuming your fuel pedal is being pushed to the floor here) with a  minimum amount of exhaust back-pressure. This actually results in a '10 lb lower than normal' boost reading. This condition happens often at high altitudes, and in below freezing weather, and is actually more fuel efficient than the normal EGR modes. From your description, this is exactly what is happening. If it is happening when it shouldn't, then someone needs to verify there are first, no engine alarms causing the EGR to be switched off, and next, to verify the temp and altitude sensor readings. Also, at the same time, remove and clean or replace the IMAP sensor. After that, Ensure the DPF 'hpa reading' is below 3.5 after the last passive regen and replace the dosing injector and its gaskets if it isn't. Lastly, keep in mind that to produce 30+PSI of boost, the newer VGT turbo's will also produce a lot of exhaust backpressure. This is to force exhaust gas into the EGR system so that it can overcome that high boost at the intake. This means that you will only see max (30+PSI) boost when the egr is actually active.

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