Domitian

The car is a 2004 1.8t, with a swapped in engine from a 2003. After the swap in August, the new K04 turbo began to gradually get activated less and less, until in December it stopped completely. Shortly after the turbo stopped working, the check engine light began turning on, but all the code said was high idle.
Fast forward to now, and I can actually notice the high idle - it is not there all the time but frequently, oscillating between 1000 RPM to even as high as 1800. The turbo doesn't work, and boost refuses to go over 8-9 psi. I replaced the N75 valve last week, and no difference, though it seems like boost slightly fell to 7-8 psi (could be imagining it). Misfire codes on cylinder 2, so given the ignition coils + spark plugs are pretty new yesterday I replaced the fuel injector for #2, but no difference.

The misfires occur as follows - sometimes on startup, sometimes after warming up the car and driving a long distance. Tends to eventually go away on its own, often when I accelerate over 3000 RPM (as if that extra pressure makes something snap back into place?). During the misfire I can smell fuel in the cabin, this disappears as soon as it stops misfiring.
We did a smoke test and the engine was clean, no leaks.

What could be going on here? My friend is beginning to call the vehicle cursed and thinking of replacing the ECU, but I dunno an ECU issue seems unlikely to me. The multifaceted nature of the problem is why we're confused -> low boost (not over 8 psi), and occasional misfires and high idle.

Domitian

I don't see an option to edit original post, so I'll add that the ECU requests up to 20psi of boost while receiving max 8.

Cybersombosis

Do a compression test and report back what numbers you are getting.

Domitian

Cylinder 1 and 4 at 150 psi, cylinder 2 at 120 psi, cylinder 3 at 80 psi. Added oil to cylinders 2 and 3 and they are back at 140 psi and the misfire is gone.
However, the high idle and low boost issue persists. The wastegate on the turbo was looking kind of loose so maybe that could be a factor?

Cybersombosis

Possibly. To test the wastegate opening pressure you will need a regulated air source set to 4psi. I use a leak down tester as it has a pressure regulator. If you find it opens up prior to 4 psi, you will need to remove the cat to get to the lock collar and remove it. Then undo the first lock nut and swing the wastegate retainer off the rod and adjust the second nut in. I found that 1 full turn is about 1.5psi. I just kept an eye on the lettering of my socket for reference. It may be so loose that you can spin the wastegate door freely or the nuts are completely missing and is flopping in the wind. Have seen that happen before.

As far as compression is concerned, it sounds like your head is either cracked or the head gasket between cylinder 2 and 3 is done. One way to test if the head gasket is leaking between the two cylinders is to have both plugs out while doing a compression test. If you are leaking between the two cylinders, you will hear a whistling sound on each revolution.

If there is a crack in the head, you will have coolant leaking into those cylinders. If you can, take the spark plugs out and have a look down the holes at the top of the pistons. If 2 and 3 are cleaner than 1 and 4, that is indicative of coolant in those cylinders which means your head is either cracked or your head gasket is done. Either one requires a head removal.