Re: Only boosting to 8psi. (A little long). - Saab 900 Bulletin Board - Saabnet.com
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Re: Only boosting to 8psi. (A little long).
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Posted by Cmyles [Email] (#1126) [Profile/Gallery] (more from Cmyles) on Tue, 16 Jun 2020 12:22:47 Share Post by Email
In Reply to: Re: Only boosting to 8psi. (A little long)., Rose quartz 87 [Profile/Gallery] , Tue, 16 Jun 2020 09:58:10
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OK, that's good info. I wonder if the noise is the wastegate rattling? I assume that you've only heard the noise from inside the car going down the road? Other mechanics in the Saab shop I worked at would connect a tiny light (LED maybe?) to the knock sensor circuit so they could actually visualize engine knock while driving (the light would flash). The APC system is just complex enough that a whole bunch of things (system defects), besides engine knock, can make it dump boost. If you have one of those many issues and a wastegate that rattles loudly you might have no engine knock at all? It might be worth tapping the wastegate and other turbo/exhaust parts with a wrench to see if you hear a rattle of the same pitch. The early cars, like your (and my) 1987 had the knock sensor cable routed in a fashion that would sometimes transmit various ordinary vibrations to the APC brain which would then dump boost. Rerouting the cable was prescribed by Saab in a bulletin. Later cars had their cables routed very differently. It also wasn't unusual to see cracked and broken knock detectors, probably due to over-tightening of the fixing bolt.

Hope that helps some.

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