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John,
I'll start off with your question about the T-shaped valve first:
The "valve/solenoid in the vertical position between the aux. air valve... and the throttle body" is the air conditioning idle-up solenoid valve. It gives more airflow past the throttle plate, similar to the auxilliary air valve when your A/C compressor is turning (A/C is a real strain on these little four-cylinder motors, so this device compensates for the load by increasing the throttle, so to speak, it just doesn't act directly on the throttle plate). It merely routes a premeasured amount of air (fixed orifice in A/C valve) around the throttle plate. You might think this would make the air/fuel mixture lean, except this air is pulled through the air filter/airflow sensor, which adjusts the air/fuel mixture automatically.
#1--You can check the modulating valve output (buzzing valve next to air filter/airflow sensor on fender wall), which controls the mixture by connecting a dwell meter to the check "plug" or connector. This test/check plug is located on the driver's (US) side of the car, growing out of the big wire sheath which leads into the fusebox. The photo below (which might take a while to load) is of my modulating valve test plug on my '84 900S. Yours should be identical. I marked "+" and "-" symbols on my plug to make hookups less of an ordeal. The "+" side of the dwell meter should connect to the terminal of the plug in which the red-striped wires end.
You can check the O2 sensor for voltage activity AFTER the car has been running long enough to heat up sensor to operating temps, about 5+ minutes should suffice. See my fledgling webpage for more info (copy & paste) http://www.geocities.com/saabman84/O2sensorplugs.html
#2--John, this idle-fluctuation, I'm told, is somewhat normal. If you are OK at 900 rpm, then I'd leave it alone. This engine should idle nicely about 900 rpm, anyway.
The 900 8-valve motors (CIS fuel injection models) should "hunt" a bit, in about a 50 rpm range around their idle speed after warmup. SAAB Tech Tom Townsend tells me this is normal as the CIS "brain" varies the fuel mixture according to the O2 sensor. (In contrast, 16V 900 cars are apparently rock-steady idlers when operating correctly.) Idle is only "steady" during the warmup period on 8-valve 900s, if ever. Even then the idle is being slowly lowered from cold, "high" idle (~1,400 rpm) to the warm idle speed (which should be 875 rpm +/- 75 rpm) by the auxilliary air valve, which is closes as it is heated, both electrically and by engine heat.
#3)--see my old post, linked below, on this same question which popped up a while back.
Hope this helps!
Micah in NC
'84 900S, 234K mi.
posted by 216.77.233...
Click here for a photo of where to connect dwell meter to read the mixture setting.
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