Low Boost 1st, 2nd Gears

[R490]

Trying to get my S4 ready for ski season, as it's the only AWD car I have now. Trying to knock out some gremlins before then.

The car has an APR Stage II tune for about 13 years and 80,000 miles now. Recently, I've changed the EGTs, coil packs, spark plugs, ICMs, fuel filter

I've noticed that when getting on it hard, I don't have but 5 PSI of boost in first and second gears. In third and above, the gauge reads about 17 at peak. I've pressure tested the car half a dozen times, to no avail. On occasion, I get a 17963 charge pressure exceeded code.

-I'm going to swap the N75 out for a known working one

-B2S1 02 sensor is getting changed this weekend

-Fuel filter banjo bolt hose is getting repaired this weekend too (It's rigged up with a piece of rubber hose right now).

My only thought is that its either the N75, or wastegate lines. But I've also heard from a couple others who have had similar issues for years, or since getting tuned.

Am I on the right track? I'm kinda stumped. I don't ever get on the boost in this car, since I barely drive it. I only just noticed it the one time I did get on it.

[LJS]

FWIW-
I think you're on the right track---
My gut says the issue is N75 related
Good idea to re-check your waste gate lines to ensure they;re not leaking(vacuum check)-- I'm sure that's not your issue.
For kicks- try disconnecting your MAF SENSOR and see if that makes a difference---don't be too hard on the auto---you'll be on canned MAPS and have some interesting AFR's and such...
let us know what happens.....

[R490]

I'll try the MAF sensor, fingers crossed its the N75.

[R490]

Replaced the N75 with a known working one.....

Boost is now much smoother and holds.

However, I'm peaking/holding at about 12-14 according to the boost gauge.

The car has an APR Stage II tune (to my knowledge), but APR program switching is not working. Makes me wonder if the car was flashed back to stock at some point. Pardon my ignorance, but most of my ownership has been spent trying to fix the car.

[LJS]

Replaced the N75 with a known working one.....

Boost is now much smoother and holds.

However, I'm peaking/holding at about 12-14 according to the boost gauge.

The car has an APR Stage II tune (to my knowledge), but APR program switching is not working. Makes me wonder if the car was flashed back to stock at some point. Pardon my ignorance, but most of my ownership has been spent trying to fix the car.

If you're holding 14PSI to red-line that's a pretty good Stage -II tune and all you can safely get out of the K03
FWIW- I had a DAZ Stage-II tune that peaked at 20psi at 3000RPM but quickly dropped to 13.5 at 6700 RPM
I can provide a data log plot if you need to review....

[R490]

Yup, and the car has been on the original k03s for 205,000 miles now. It's been tuned since 2011 and about 100,000 miles or so.

I'm not positive the gauge is entirely accurate, when it was overboosting, it was reading super high and was all over the place... I'm also not sure what the tune is doing, since it is supposed to have program switching.