Boost bleeding off issue
 

Rundown. 88 Turbo S, 42k on the clock, just installed Vitesse DME Chip, 3 bar fpr, black knob MBC, and Tial 38mm wastegate. When first installed, I was set at 17 psi and holding till redline. 1k miles later boost is now dropping down to about 13 ish psi by redline. I took off the DV valve and tried blowing air through it but couldn't get it to leak at all, so I don't think it's the DV valve, at least not yet. Any pointers?

Try the 993 part # blow off valve as they have brass internals yet look exactly the same as the 951 stock diverter! All 951 diverters are prone to only hold 12 psi or so after a few years of oil sitting on them! Other than that its really just the usual hose clamp and rubber boot checks which Im sure you have done already! I cant imagine your wastegate is bad?

Yup, looked at all the vacuum lines that I ran for the MBC, they are all on and zip tied still. The MBC is still responsive, if I turn it up I get more max boost, turn it down it goes down. So I don't think it is anything with that part. I'll try picking up a Forge DV or the 993 part you suggested. I'll probably need one down the line anyways as mentioned. The one on the car now seems good still.

Would a slight exhaust leak cause something like this? I don't hear any leaks, but I didn't replace any gaskets when I put the Tial wastegate in.

Just replaced the DV valve with a Forge unit. It has the green spring in it which is rated for 6-8 psi. Still losing boost, bleeds off to about 10 psi at towards redline. Full boost is set to 16-17 psi. Should I try a higher rated spring in the DV valve?

wow i have almost the same set up as you with the wastgate boost controller and bov but mine is the forge 006. are you sure theres no vac leaks because usually thats what happens to me when a line pops off.

No, vacuum looks good. I put in a stronger spring (yellow) and now only boosting to 10psi without touching the manual boost controller.

I am going to turn up the boost controller to get back to 16-17psi and see if it holds.

Hopefully the inner liner of your DP isn't collapsing, as I believe that would cause the symptoms that you are describing.

I am going to start replacing all vacuum lines with new silicone line.......... :(