Built motor, 8.8:1 compression
Powerlabs single turbo kit, Greddy GT35R .82
Tial MV-R (first release), new 10lb 44mm spring, manual controller
Equal length headers, 3" turbo-back, Tomei exhaust
UpRev GT MAF, Tuned on Osiris

I did have this same problem last year on a 6lb spring controlled to 10lbs. It would boost to 10lbs right away, then bleed off to around 7ish. We suspected a weak spring or bad diaphragm. I checked the diaphragm and it appeared fine, and now I have a new 10lb spring and still doing it. Using a new boost controller this time around also, so don't think that's it.

Built motor over the winter, got car tuned yesterday and having same problems with boost loss. Will boost to 20psi right away and hold steady until ~4600-5000rpm where it will drop and fluctuate 14-15psi for the rest of the RPM range. We did a leak test, everything held fine up to 18psi (didn't go any higher). Wastegate reference is the original Powerlabs location, right after intercooler. My next test is moving the reference to the manifold, and purchasing a elec boost controller. If that doesn't help then might look into a new wastegate. Looking for some more ideas or if anyone has ran into this problem before.

 

I never liked the manual boost controllers. I would suspect it has something to do with that.

Will it boost 10lbs(with the 10 WG spring and no boost controller) and hold. If so the MBC is not working properly

 

when just using spring pressure it was consistent at 11.5psi. I am going to order a EBC either today or next week.

 

If you don't plan on going higher than WG pressure just get the proper spring. Although the E boost controller does help for spooling

 

So after a lot of driving this weekend I have noticed something I wanted to mention in hopes it sparks ideas. If I don't give 100% throttle, but stay at 75-80% I will see 18-20psi without dropping, but if I give full throttle I will see a drop in boost.