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Preface, I have a 1999 viggen with 110K miles.
I worked with Jak over the phone and he helped me solve the same problem. I was driving myself nuts over this for about 9 months. I changed the bpc, dic, maf, vac lines, plugs, by-pass valve (the thing that connects to the bpc that dumps pressure), even swapped in a new ecu, took apart the wastegate too. Still only 60% boost.
Heres what to do. First, Get a smbc or direct connect the turbo vac lines to build boost. Becareful because when you direct connect the turbo, you can build boost super fast with no protection to stop from overboost. Crank it up to see if you can build boost. If you can't build boost you have a bpc issue or something else.
If you can build boost, you have another problem...here is the real secret. Connected to the throttle body there is a hose that connects to the evap purge valve via a hard plastic vac line. There is a T-connection in this line and the other end of the hard plastic vac line is the bpc. There are 2 check valves in this vac line from the throttle to the bpc. Take the vac lines off and try to suck and blow through them. If either check valve does not function like a proper check valve should...you've found your problem.
Its like $160 part from the dealer. Only place I could find it.
The faulty check valves flow air into the bpc when there should not be air in there. Your bpc is not broken, its just getting air at the wrong time and it screws up everything else.
I spent 9 months and 3 saab indies to find this answer. Jak Stoll is the man. Let me know if this helped. Oh and if you haven't already, install a real boost gauge. It really helps analyze stuff like this.
-Adam
posted by 12.30.177...
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