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This sounds exactly like a problem I was having - especially the part about the boost very suddenly "jumping" into the red. Just bear with me while I work through this:
I believe that your problem is related to your bypass valve.
I have a Forge bypass valve which, when I first bought it, gave me this kind of problem (mostly in second gear) EVERY SINGLE TIME I ACCELERATED:
Push the throttle...car lags w/low boost...push/stab the throttle harder...suddenly pops into high boost.
Problem was reduced or eliminated in 3rd-5th gear.
After several week of troubleshooting, I finally discovered the problem:
The spring in the bypass valve was too weak! Since the Forge valve can be dismantled, I opened it up and put in several thick washers to further compress the spring that holds the diaphragm closed. The problem was solved INSTANTLY!
Now then...the only time I ever see the problem return (maybe once or twice a month) is in a situation like you describe, crusing at 30mph then applying throttle.
I believe it has to do with the position of the bypass valve on the metal intake pipe, relative to the vaccum line that actuates the bypass valve, which is fed further down the line from the intake manifold itself (obviously it is a long thin restrictive line).
Cruising at 30mph puts the whole system into a nice steady vaccum. Stabbing the throttle creates a pressure wave which reaches the bypass valve and pushes (quite hard really) on its diaphragm which is already open due to the vaccum which existed in the whole system a fraction of a second before. At the exact moment that the pressure wave his the bypass valve, there may in fact still be a vaccum the line coming from the intake manifold. At the very least, the pressure on the bypass valve is significantly greater than the pressure in the line.
Now...given the fact that at this point you're losing most of your boost pressure through the open bypass valve, very little boost is actually making it to the intake. All the boost is just getting sucked back into the turbo.
So long as you stay on the throttle (once this condition is created) it will be hard to break the cycle UNTIL enough pressure makes its way to the vaccum line (combined with the force of the spring) to overcome the pressure on the diaphragm from the intake, then WHAM! the bypass valve slams closed and sends ALL the pressure to the intake where it belongs.
It should therefore seem obvious how increasing the compression on my bypass valve spring solved this problem. However, the stock Bosch bypass valve is not adjustable. :(
In your case I see two possible solutions:
Get the higher pressure "viggen" bypass valve (or an adjustable aftermarket one) or, as a trouble shooting measure, reverse the orientation of your bypass valve so that the pressure from the intake pushes perpendicular to the diaphragm, rather than against it. Don't worry, I (and many others) have tried it both ways and it will work either way.
So why doesnt this happen in 3rd-5th gear? Well... I believe that it's the same principle that describes the lack of significant boost lag in those gears. I think that this is intutive, and you probably understand what I mean...
I have to get back to work now...
Good Luck!
-Kyle
'91 9000T
posted by 64.65.195...
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