This is what happens when incorrect airpath causes air turbulence to the MAF

Owner cuts the shroud behind the aircleaner.
This allows airflow to swirl that causes the MAF to improperly report the true airmass.
Tuning the PCM as we see below via scanner data that the fuel trims are as much as 25 % too lean when in fact engine is running only about 10% lean would cause the engine really to be way too much rich but tuner is going by the reported 25%

If owner never found this out and PCM was tuned for this false lean condition not only would the fuel mileage drop, cause of excess carbon buildup, surging and non crisp response of the engine

This causes hugh issues for performance and confuses the owner why the powertrain is sluggish when they believe the fuel trims are that lean when they are not.

The top graph is what the engine fuel trims are being reported and bottom graph is when I had customer cover over the hole they put in the radiator shroud.
Proper path if doing this is to have some type of box ( ours is called the Icebox) surrounding the filter which generate air pressure and prevents air blowing right on through filter and causing non smooth airflow through the MAF

[Linked Image]

Team ZR-1
True Custom Performance Tuning