95 850 Turbo - boost to manifold, but power cuts out?

[wixis95]

This issue started out at as only getting 2psi (per aftermarket gauge) when boosting, but when pulling the hose to the wastegate, I could get up 7-8psi, albeit it was somewhat stuttery and not smooth. This being the case, I opted for a Pierburg TCV to replace the stock one - no change.

I went ahead and checked/replaced all vac lines with new silicone and clamps - I checked every single one, including the one from the throttle body tree to the gauge, so I know my intake readings are good. Checked all boost pipes (new, IPD RIP kit) and they are good. No play in the turbo shaft. Wastegate is good/holds pressure.

After doing that and swapping back in the old TCV, I can actually get to 6-7psi now, but around 6psi or ~3k ish RPM, it's like the power drops off. It will hold 6-7psi boost at that point, but it's like the ECU took away some fuel or spark and it won't make power. There are no CEL or codes.

Here's what's been done in the past year or so: new CBV diaphragm, new fuel pump and filter, new fuel regulator, new MAF, coils/plugs/wires (gap on plugs are good), all vac and turbo lines, air filter, PCV system, timing/water pump/belts (this was done a while ago and timing is definitely right), upstream O2 sensor, throttle pos sensor.

Totally at a loss and very annoyed lol - any help is greatly appreciated!!

[Sveedy]

So did this problem just pop up on its own, or was some work recently done that might have caused it ? Might be your intercooler.
I guess I'd try a smoke test of the whole system to see if something is leaking that you can't easily see by eye. I had a crack in the plastic part of the stock intercooler that I found that way.

[wixis95]

No recent work, no. Just out of nowhere. The reason I'm not currently suspecting a boost leak is because I know it's capable of boosting to 7psi with the wastegate hose undone, but when it is hooked up, the ECU or something is causing the TCV to limit it to 3psi. In my (very amateur) opinion, to me that says the ECU is reading something and limiting boost on purpose, and I'm thinking that same thing might be at play when the wastegate hose is hooked in, thus not letting it boost past 7/rev past 3k.

It is also getting noticeably bad gas mileage, even when I stay out of boost and just drive gently.

It runs totally fine outside of this issue btw, smooth idle and no rough starts/shifts at all.

[Sveedy]

Well I'm at a loss. Logically I'd suspect what ever might fail under pressure, so since things seem to build more with the wastegate disconnected, then maybe it, or the CBV ( only other thing you haven't mentioned ) has gone bad. But I don't know if that would cause bad mpg. I don't know what the failure rate of these ECU's is, but I can't see it being very high.
You might try posting this over on Swedespeed as well. There are a few more performance oriented guys over there.

[wixis95]

CBV is new as well, but I did double check for leaks recently. I'm totally stumped too - I almost wish I had a CEL so I could have some direction. Thanks for the recommendation - will do!