To effectively eliminate the p1188 code, DCX needs to incorporate both a BARO and TIP sensor - and not rely on a solenoid and excess hose to switch a single sensor between both duties.
Putting in a pressure-reject or vacuum reject check-valve would solve the overall code throwing part of the problem, but would eliminate half of the data that the PCM is anticiating from the stock one, and this may result in performance inconsistency.
Your friend's fix is a good fix to eliminate the code, and indicates a good deal of thought went into it.
The problem seems to come from the fact that the sensor is toggled between duties of Barometric pressure (when not under boost) and Throttle Inlet Pressure (When under boost).
It is a mechanical re-routing of pressure/vacuum that is achieved electrically by it's associated solenoid (on the passenger firewall) that is the root of the problem.
The time delay created by the closing of solenoid contact and the difference in time for a change from pressure to vacuum or vice-versa was apparently not factored into the PCM programming, it becomes confused when it sees pressure and is anticipating vacuum (or vacuum when it is anticipating boost).
Adding a MBC further complicates this with additional hose and a bleed valve. They both introduce additional time delays.
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