So I was driving home to my mom's house and at the beginning of my 4.5 hour drive, I found that my car was overboosting. WAY overboosting.
Normally when I go into the boost, it stops before 3 o clock. Today, it would spike up to about 5 o clock, which is BEYOND the yellow band.
Needless to say, I was very careful driving.
I also noticed it doing the following:
When driving with a steady foot near the boost range, the car would creep into the boost, then creep back to vacuum, then repeat in a cyclical pattern.
When boosting to go up a hill or whatever, the boost would spike (if I was careful to not overboost) and then come back down. For example, it would reach up to 4 o clock and then drop down to 1 o clock on its own. (The car's power would also trail off)

What I thought was that I had blown a hole in my CBV diaphragm. I'm running a Mitsu TD04 turbo. The CBV is separate from the turbo and is fed by a separate pressure/vacuum hose (the hose is attached to the turbo directly by what looks like a rubber vacuum hose covered with braided fabric).
I stopped twice - once for gas, and once to check things out. After I checked them out, the car stopped over-boosting.... for a while. Once I had been driving about 15-20 minutes, it was doing it again.

Anyone, please give me suggestions on what to check for. Everything's hot right now and I'm gonna eat dinner. I can check it out tonight or tomorrow, but it's traveling season and I'm supposed to drive.
I'm thinking check for vacuum leaks, check for the hoses feeding the turbo to be old and crackly (they're maybe 3 years old at most), see if I can move the CBV actuator rod myself.
I don't understand how the car can be doing this. Maybe the hoses heat up and that one particular hose (fabric covered and turbo to CBV), which I DID NOT replace is bloating and leaking boost. Or the diaphragm is leaking and once it hits a high enough pressure, it's enough to open the CBV some. It also seems heat related.
Maybe some kludge is built up around the CBV and sticking it shut until the turbo hits a higher boost.
Maybe the CBV is completely stuck shut for whatever reason, and the car is retarding the timing - causing the drop in power and a connected drop in boost - ie the car loses power so the boost drops, not vice a versa.

ARGH. help.
Dinner.
Will
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1990 740 Turbo, on its way to stock specs, maybe beyond

Update:
Checked out the turbo to wastegate actuator hose and found it was semi-plugged with what looks like coked oil. Also looked like the same thing was in the diaphragm nipple.
I cleaned it out and replaced the hose (the turbo side was brittle and heat-bonded to the nipple) with some fuel/emissions hoses. I'll have to get a legit volvo hose, but it should work for now. The ID is the same, and it's reinforced, but the OD is huge and I can't fit the clamps on it.
The hose didn't seem to be leaking.
It looks like the TD04 is easy to disassemble (one clamp and splined for alignment), so maybe I will pull it apart and see if the wastegate is all gummed up.
If so, easy fix, if not.... I think the diaphragm must have a hole in it.

Thanks guys,
Will

PS I think the fuel pump cutoff switch is missing (or moved) because the engine was swapped out at some point... yaaaay frankencar :|
Happy holidays :D new brake pads tonight.
--
1990 740 Turbo, on its way to stock specs, maybe beyond

Ok, I checked out the turbo pics and see that the CBV and wastegate are not the same thing. Also read up on them a bit, but maybe I'm dense.
Seems like the wastegate is a bypass that limits the amount of max boost by opening at X psi. The CBV vents pressurized intake air from the compressor output back into the compressor intake and only really exists to take strain off the compressors.

So then the only thing that could be causing this is the wastegate not operating.
I saw the CBV moving today when I revved the engine.
I'm gonna get up early and check the turbo->wastegate hose. I bet (and hope) that's it.

-Will
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1990 740 Turbo, on its way to stock specs, maybe beyond

Hey Will,

Your car is supposed to have a high boost cutoff that, if operating, will kill the fuel pumps if the pressure goes too high. There is a small switch, usually, in front of the coolant tank with two yellow or yellow and black wires in it. They are the ground for the fuel pumps. You need to buy a good boost guage also. The Volvo gauge is a piece of crap for knowing what the boost is doing. I believe the factory setting is 6psi, and I have run beyond 9psi without killing the motor. If the waste gate is locked shut (carbon buildup in one of the lines probably) the boost could go past 15-20psi on full throttle. It should run like a booger though for a short while.:)

REgards,
--
Will Dallas, www.willdallas.us, www.willdallas.org, www.willdallas.com, www.dallasprecision.com 86 245 DL 222K miles, 93 940 260K miles, 88 765 GLE 152K miles, 88 780 246K miles, 93 Buick LaSabre 119,000 miles

hello
i would check and see if the wastegate is stuck or unhooked. maybe the wastegate actuator is gone bad.
or check the small hose that goes from the turbo compressor to the wastegate actuator, i replace them from time to time.
seems like the wastegate is not dumping the extra boost.
the cbv is there to prevent overboost, on my car it seems to open up at 16 psi and recurculates the extra boost back through the turbo.
shouldnt be too big a problem.
i think the 1990 740 turbo has a overboost cut out switch but im not sure.
good luck
mike

OK. I saw that the actuator rod was connected earlier today.
CBV=wastegate, right?
And :O 16 psi? that seems astoundingly high. I thought the stock boost was 7 or 8.
I think the overboost switch is 760's or turbo+ models. I think it's a fuel pump cutoff switch..?
Thanks. I guess I just hafta poke around. The more I think about it, the more it seems like the hose from the turbo to the actuator is rotten.
-Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond

The CBV is a valve that connects the intake pre-compressor to the intake post-compressor. Its also called a blow-off valve among other things. It's there to prevent compressor surge, and vent excess boost back into the intake. It is different than the wastegate.

The problem sounds like the wastegate to me. This is a valve and actuator (actuated by intake pressure) that lets exhaust gasses pass around the turbine and into the exhaust, so their energy does not contribute to spooling the turbo and creating more boost. This is the primary means of controlling boost, and if you're overboosting, then this is the first place to look.