1985 745t m46

(opening note: this is a long post, and I apologize in advance. Most of it is just my take on possible problems. I just want to make sure all relevant information is available at the forefront.)

Over the past few months I've been experiencing a problem where my brick will exhibit a stumble that is inversly proportional to the amount of boost I apply. Basically I can slightly enter the yellow area of the boost gauge without any real problems, but as soon as the needle ruses towards 11 o'clock or greater the car starts to lose power. It's hard to describe, but it basically feels like the car is trying its damndest to boost but is held back somehow. I can hear the turbo spool up as it reaches 2/3s into the black, can feel a slight acceleration (very slight) as I cross into the yellow, but at medium to max boost it sounds a bit like a muffled vaccuum sucking on a pillow. You can also recreate the problem slightly by putting your top lip over your lower and blowing while humming in a low octave... ;)

I should note that the wierdest part about all this is that the idle is SOLID. It vacillates a tiny bit when idling before it has been run (like 50-75 on the tach), but I see that as a factor of an older engine and not a sign of a physical problem. When its really cold it will sometimes take a little bit to settle down to an idle tach level (sinks from 1400 down as it warms up), but this generally only happens in seriously cold temperatures like we've been having recently.

Below is a list of most of the symptoms I have gathered:
- stumbles as indicated under moderate to heavy boost
- fuel consumption down from 23-25mpg to 17-19mpg (I think 17 is likely when I try to boost more frequently during a trip, but I don't know for certain)
- has more power at higher rpms than anywhere else
- does not like hills
- performs even worse when very cold (boost or no boost)
- Higher gears (4th, OD) or the weakest of the bunch unless running at 2700+ rpms.

As for the turbo itself, I can hear it spool up and am not tossing or burning any excessive oil, so I think it's sound. The cat has been replaced in the last year with a new volvo unit and the exhaust manifold was replaced at the same time by an indy pro, so I don't think any of those components are involved.

I posted on this about 2 weeks ago and hypothesized that I might have been getting some bad or low octane fuel from a discount station (I pump 93, but who knows what they were selling). However, while this may still be a factor I've gone through a number of tanks since then and should have blown through any of that fuel by now (including any in the filter).

Below are a few ideas I've been kicking around:

Air Mass Meter:
Always a likely culprit when dealing with a possible fuel/air mixture problem, I think this might be involved somehow. However, the fact that the idle is fairly solid would seem to undermine this as a factor. Wouldn't the car run just as badly at idle, at highway speed sans boost, and at boost?

Plugged Cat or exhaust:
Possible, but I had the cat replaced 11 months ago and trust the mechanic that did the work (and the unit he put on was brand new). Anyway, it's unlikely its causing the issues. I doubt its the exhaust since I haven't been throwing oil and it was working fine not too long ago.

o2 sensor:
I keep coming back to this and wondering if it might be involved. Could the o2 sensor be leaning out the mixture under boost for some reason? Doesn't the boost only nature of this porblem indicate it can't be the o2 sensor? Does anything change in the exhaust under boost that the o2 sensor might react to?

knock sensor:
Like the o2 sensor, I can't help think that either there's a wire problem or the KS itself is dying. If it was falsely detecting knock under boost might it lean out the mixture and cause the performance loss I'm seeing? Wouldn't it be more likely that a faulty KS would be apparent all the time than just under medium to heavy boost?

ECT sensor:
Might as well round out with the last of the three. I replaced the ECT sensor with a new one about a month back, but it didn't have any effect. I guess it could be a wiring issue and I shoudl cut back teh sheath to examine it, but like the aformentioned two sensors a faulty ECT should be apparent all the time. Addditionally, in temps like I'm experiencing now I don't think I should even be able to start the car (-20f is way below the default ECU responce to a faulty ECT signal, right?).

Fuel delivery system:
I've replaced almost every element in the line, including the sender, pre-pump, sock, filter, FPRelay, and FPReg in the last year. I also ran some cycles of cleaner through awhile back (not together and well apart), and cleaned teh airbox and replaced the filter about 10months back. Basically I don't think it's a deliver issue.

Coil:
A friend of mine who knows a bit about cars mentioned that maybe the coil was weak or something (he was searching for any possibility). I think he mentioned that under higher compression (boost) it would take a stronger spark and thus a stronger coil charge to ignite the fuel in the cylinder, and that a dying coil or wires might cause the problems I'm seeing. The wires all seem solid (distr. to plugs, coil to distr.), but maybe the coil itself is weak? Would this cause the problem I'm seeing?

Pressure side leak:
This seems like the obvious possibility; a leak on the pressure side of the turbo that is allowing measured air to escape and thus reducing the amount actually introduced into the mixture (leading to more too much fuel and my stumbling). I've examined the AMM to turbo, turbo to intercooler, and intercooler to TB hoses pretty closely and can't find any leaks. Is there anywhere I might look that is often missed? It has to be something that would only be apparent or leaking under medium to high boost since it idles fine.

CBV:
Someone (I think John Sargent) mentioned a faulty or leaking CBV as a possible culprit. I took a look and it seems solid (I need to examine it further but its too cold right now). However, wouldn't a faulty CBV, say one that was open under vacuum, have to be REALLY open to allow anything other than a very slight amount of air to escape? I know for certain mine DOES close under vacuum, so I have to think that it wouldn't be allowing enough air to escape to have the kind of effect I'm seeing on my performance at medium to heavy boost.

Bad Rings or Head Gasket:
As with most of teh other problems, I would think that either of these problems would exhibit obvious symptoms in other locations. My oil and coolant are both clean and sound, I'm not tossing any oil, I'm not smoking once the car has warmed up (always a little white at first start on cold mornings) and the idle is solid.

Thanks to any and all that take the time to read through all this. Unfortunately I can't do much direct analysis since temps here are currently around 0f (-10f to -20f with wind chill), so I'm reduced to trying to whittle down my enormous list of potential problems.

Any thoughts are appreciated.

Thanks,
rt