Rundown. 88 Turbo S, 42k on the clock, just installed Vitesse DME Chip, 3 bar fpr, black **** 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?

 

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?

 

Do you have any oil leaks? If so, check the AOS seal. This usually goes when you increase the boost and if it has never been changed (age related).

 

Have you tried to re-adjust your MBC?

You either have a dead MBC, bad WG, or boost leak... ON boost the DV either holds or doesn't, but it shouldn't be causing bleed off from peak boost... even if it were bad.

 

My vote is for a Vacuum leak. Have fun

 

vac. leak plus +1 with 40k miles bet under the hood has not been tampered with much meaning old lines and gaskets.

 

Check all the pressure AND vacuum hoses/connections.

 

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.

All the vacuum lines look good. Idling I am getting -20 pounds of vacuum. In gear coasting about -25pounds of vacuum. Wastegate is a Tial and is brand new. The lines I ran to it from the boost controller I have put hose clamps on. Zip tied the rest of the lines I ran for the MBC/Wastegate setup in the engine bay.

 

No oil drips under the car if left parked for a while. It's pretty dry. The back of the head does have some very slight seepage. It's been pretty good about not burning any oil either. I only have to top it off so far every 1k miles. It goes from full to about 3/4 on the dipstick.

 

The MBC responds accordingly. The Tial wastegate is brand new so hopefully not that. I didn't hear any kind of exhaust leaks after I finished putting everything together.

 

Is it making and holding 10psi or making more boost and rolling off to 10psi?

 

Right now, it is making 10 psi and holding until redline. I didn't have a chance to drive it after I turned the MBC up a bit to get back to ~17psi. Will post later after my drive home, hopefully not to much traffic.

 

cranked the MBC up a bit, but didn't really make a difference with the yellow spring in the DV. Put the Green spring back in and I am back to spiking 17 but dropping down to 10 psi by redline.

 

I looked over all the vacuum line fittings and all the rubber fittings look to be in good shape. No cracking on the rubber boots, are there any fittings/lines that are not easily visible poking around the engine bay that I may have missed? Someone mentioned the AOS, but I am not leaking any oil and the motor is barely burning any up. Thanks all for your help!

 

Well, it seems the more you mess with the DV and or the hoses in, out and around the DV is what is causing differences. So I would look in and around the DV for your problem.

The leak is small, or what ever is leaking, is holding 17psi for a bit and then the leak opens up and then bleeds boost off. 10-12 psi peak is usually the sign of a small crack or cut in a boost line.

Many leaks you will not see, only hear with a pressure test or with soapy water to look for bubbles and also can be a split in a hose under a hose clamp.