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I was just cruising through Bulletin board and I noticed this message stream.
From what I can discern from the various posts, I too had a very similar problem with my 95 900 SET.
From time to time, especially at lower rpms in the higher gears the car would 'over boost' and cut-out. More normally the boost would rise higher than normal, then cut back dramatically (then repeat if I was still accelerating say at 55 in 5th at 3/4 throttle). I too changed the spark plugs etc and found moderate improvement. I had the dealer look at, and they came up with nothing.
I figured that the problem was with the boost control, either mechanical (sticking waste gate or incorrect base boost) or electronic (APC Valve or some sensor).
To test that theory I simply connected the vacuum (pressure) tubes (one of the three to the APC control valve) from the pressure side of the turbo directly to the waste gate actuator, and plugged the one from the waste gate back into the APC control value to prevent vacuum leaks. This in effect takes the electronics out of the boost control picture and causes the turbo to produce only base boost (which is very conservative, 'mid yellow') pressure. In this configuration the (limited) boost was smooth, steady and reliable.
So I figured the problem must be electronic. I bought the trionic book and tested all the sensors etc, but found nothing not to spec. That led me to the only component which I couldn't really test. The APC control valve. Unfortunately at $210+ dollars it was too expensive of a part to just buy on a hypothesis, so the problem remained.
Just recently the APC control valve failed completely (though the Saab dealer will tell you this never happens). When it failed the car had no boost control, and bled off all the pressure. The waste gate would never open, and the car would over boost at anything more that the most moderate acceleration.
When I broke down and replaced the APC control valve with a new one, all the problems went away and the boost control worked like new. APC control valve, thats the ticket.
What I should have done (and recommend you do) is find someone who has a NG900 or similar vintage 9000 and 'borrow' their APC control valve. It is very simple to remove, and the test would only take a few minutes once you put it in. Heck, if you happen to live in the Seattle area, let me know, you can borrow mine...
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