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Having just fixed mine, you may like to find my thread from last weak. Search for "base boost" and look for Andy (West London).
As I only bought my first Saab Turbo 2 weeks ago and finding that the car stopped fully boosting a few days later, I have scanned these BB messages on the topic. I am reasonably technically competent (I restore vintage Lambretta scooters as a hobby), this whole turbo troubleshooting has been a massive learning curve, so as I fixed mine only an hour ago, I'll summarise it below:
Although there are many things that can limit your car to what is called "base boost", which shows itself as when you put load on the engine, ie accelerating, your boost gauge goes up and just seems to not go any higher than straight up (just into yellow band). This is called "base boost". Your Engine management system (APC)is designed to (among other things) protect the engine if it detects a fault.
First thing to check would be to look at the BPC solenoid, which is the black plastic object with 3 vacuum hoses attached to it. This valve is attached to the top of the right hand side of the radiator when standing in front of the car. Remove the middle hose and see if you have full boost (be careful as the purpose of this valve is to open the wastegate when the APC detects that the turbo is at the limit of it's boost. There are other protection devices which will kick in if you overboost into the solid red band, so don't be alarmed if the engine makes a noise and shudder if you do floor it, this is just the protection kicking in. If you DO have full boost with the middle hose removed, it is likely that your BPC valve is faulty. These are expensive, but some have removed them (mark which hose goes where first)and cleaned them with switch cleaner aerosol or brake cleaner.
If (like me) you still have base boost with the middle hose removed, check that the wastegate rod hasn't become detached. This is impossible to tell from above, but if you follow the middle hose from the BPC it ends up on a rounded metal thingy about the size of a yoyo.
If you slide under the car, you should see that a rod comes out of that metal thingy about the size of a pencil and SHOULD be attached to a lever. Apparently, these commonly slip off, so pop it back on. There is a hole in the rod that you can slip a piece of wire through (a coat hanger might need to be sacrificed for this)to hold the rod in place.
Car should now boost into striped red band.
If it is neither of these things, you might want to check the valve that is attached between the vacuum hose that goes from the rocker box cover (at the back of the engine). This looks like a fuel filter but is actually a one way valve. It prevents pressure build up in the engine which can get nasty.
To check it, remove the hose that connects between this valve and the inlet manifold. Disconnect it at the manifold end and suck on the pipe. You should be able to suck air out of the rocker box. Now try blowing as hard as you can. You should feel total resistance. If not it needs replacing, but good news it is cheap $20ish.
If none of this works, there are other things you should check, such as hooter valve (search "hooter" on this BB.
Sorry for the tome, but hope this helps.
When you do get things sorted, the APC will benefit from being "adapted". Easy way is to disconnect battery (make sure you know radio code first)and leave it for a couple of hours or more.
When you reconnect, the APC will have "forgotten" your driving style and will need to learn it. It will do this naturally over time, but you can short cut this process by performing an "adaption run". This is best if you put a heavy load on the engine ie going up hill with a car full of "big boned" guys :-). You need to go full boost for at least 3 seconds within a range of revs around 3K. Search "adaption" on this BB for more detail.
Sorry for the tome, but hope this helps.
Regards and good luck
Andy (West London)
posted by 82.43.12...
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