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Q: Question About changing the Diameter of MAF housing

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I'll try to keep this as simple as possible.

I have an aftermarket intake with a MAF housing SMALLER than stock.

After realizing that the intake was causing me to run rich, I decided to experiment with ways to "offset" the rich condition.

I drilled a hole in the aftermarket intake AFTER the MAF sensor hoping that this would create a vacuum leak and cause a more "Lean" Condition.

After taking a bunch of data logs, I discovered that having an open valve AFTER the MAF housing (Air being Unaccounted for by the ECU) actually caused me to run slightly MORE RICH.

Please explain how this can happen.
My car has an automatic transmission.

Hi there. The oxygen sensor still detects rich/lean, regardless of how the mixture is calculated, and sends "lean it down" or ’richen it up" signals to engine management. If engine management is out of range on the combination of signals it knows how to operate with, it could be too rich, or worse yet, could default to an "open loop" mode, where it essentially gives up on rich/lean management (such as during engine warm-up where the engine is, by design, running pretty rich). Working with modified OEM systems that are not really adjustable is hard, and can easily produce the results, and challenges you are experiencing.

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