B8 S4 stock

I need help with this one. After about a year of 21Mpg I changed to the K&N filter and suddenly we dropped to 8Mpg for a full tank and now (stock air filter restored) we are hovering at 16Mpg. No changes to commute, no spirited driving, no Sport mode.
I cannot find any error codes other than the usual EVAP 441.
The car is burning considerably more fuel than it did before since we run the K&N for that one tank of gas. Something broke during that time and I need your help figuring out what.
I already checked the three MAP and they all show the same pressure when the engine is running so either they are all toasted or they all work?

Symptoms:
- rough idle
- excessive Mpg
- no CEL
- no VAG faults

Note: do not get side tracked by the Mpg vs. spirited driving. Trust me, this is not user error.

Help please.

Fast400

MAF reconnected after swap?

Hagarr

Sounds to me like a dirty MAF.
I would clean the MAF and disconnect the battery to reset everything.

Brian

RobC

Back in the B5 days on a tuned car it was recommended not to use K&N because the increased flow rate of the intake would cause the filter's oil to leech out and coat the MAF sensor.

Be careful cleaning the sensor because it usually has a really fine wire. I seem to recall people soaking the element in isopropyl and letting it air dry.

B8 S4 stock

The B8 S4 not longer use a Mass Air Flow sensor. It instead uses 3 Manifold Air Pressure sensors. The technology is different and, in theory, MAP cannot foul. That said, I agree with the overall comments, it might be work looking into how to properly clean the three MAPs and see if it helps. Will post an update.

B8 S4 stock

Correction: Manifold Absolute Pressure = MAP

NWS4Guy

Too much oil on the filter, can indeed cause problems even without the MAF (MAP is correct though)