Stage 1 power loss (not a vacuum leak)
 

Hi all, was hoping to get some ideas of which direction to go in in regards to my early build 2001 6spd.

Vehicle currently has 89k miles on it. Bought the car bone stock at 83k miles. Replaced the air filter, DVs changed to 710Ns and ran fuel system cleaner. Car ran perfectly.

At 86k miles I had the ECU flashed to APR stage 1. The difference was unbelievable. Power came on at 2200, at 3200 it would push you back in the seat and at 3900 your head would hit the headrest. Car ran perfectly for about 2 weeks.

About 3 weeks ago I noticed I wasn't getting quite as much power, and especially that the car not longer would "peak" at 3900 rpms. Over the course of the last few weeks, it has seemed to lose that high end extra boost almost to the point where it feels just slightly faster than stock.

No CEL on and vag-con is showing no stored codes at all. Funny thing is, the car runs perfectly fine, its very smooth with no misfires at all, just doesn't seem to be making the same boost and power.I checked all vacuum lines and there doesn't seem to be any leaks anywhere. My brother (who is an ex volvo mechanic) also checked and said the same thing.

I am doing the timing belt in two weekends with a full blauparts kit.

First off, I will also be changing the spark plugs as I have no service history at all on the car, and I am sure they need to be done anyways. Normally I just use the Bosch FR 7 LDC+, but after talking with the shop that tuned the car and reading posts, I was going to go with a plug one temperature lower (any recommendations? Anyone here used NGK PFR6Qs?).

What else would be good to replace/take a look at? Shop that tuned the car recommended I change the MAF sensor (although I have no reason to believe its bad) and the N75 wastegate bypass valves and then see what the car feels like after that and the new plugs.

The issue doesn't seem like the plugs to me. It seems like that car is letting go of boost before it used too. Only other things; It might be in my head, but I feel like the car actually has less power now than even when it was stock below 2200 rpm and I have noticed that sometimes when I start the car cold it idles at 1400 rpm for a few minutes instead of 800, but I am not sure if it did that before.

Any direction at all would be a huge help I really appreciate it. Thanks.

Not exactly sure but something to just take a look at is your throttle body boot. If you have a small tear in it than it could be working fine under normal driving but when you apply a lot of boost it could be stretching and opening up a rip and in turn loosing all your boost through it. Like i said im not sure exactly but its something to take a look it. Wouldn't be the first time its happened.