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Mark,
My Mom's 1990 900 base did antics like your 400-500 RPM and die scenario. It never did the 1,500-3,000 RPM thing, though. Her has been diagnosed affirmitively as the AMM (black plastic cylinder with wide electrical plug between air filter and throttle body).
The engine's computer (LH Jetronic 2.4 version in these cars) relies solely on the AMM to dispense gasoline to the engine WHILE IT IS WARMING UP. After the engine warms up, it "listens" partly to the AMM signals and mostly to the oxygen sensor input to dispense fuel.
Or...
1) the AIC (automatic idle control) valve could be dirty/wearing out,
2) the (CTS) coolant temperature sensor could be faulty,
3) the TPS (throttle position switch) could be misadjusted or broken.
Best bet is to try disconnecting the AMM and see if it runs better during warmup period--LH-Jetronic brain will use a preset value and Check Engine light may illuminate. Even better is swapping out your AMM with a known good same-type AMM (SAAB used different AMMs for 1986-88 900S & 1985-88 T's).
If that's no help, then try disconnecting the plug to the blue-bodied CTS, which is located between cylinders 2&3 facing driver's side, just below intake manifold runners. Again, the LH computer will use a preset value when CTS disconnected.
Also, the CTS can be tested with an ohmmeter. From the Saabnet FAQ pages: "Cold Engine = 5,800 ohms @ 32°F (0°C) to 2,600 ohms @ 68°F (20°C)
Warm Engine - 320 ohms @ 176°F (80°C) to 180 ohms @ 212°F (100°C)." Remember, those figures are the COOLANT temps, not ambient air temps.
Check Bentley 16V manual, the Townsend Imports website, or this website's wonderful Frequently Asked Question's OG900 LH section for more info.
Hope this helps!
Micah in NC
'84 900S, 228K mi.
'89 900S, 109K mi. (project car)
posted by 216.77.236...
Click here to see Saabnet's FAQ on LH-Jetronic troubles.
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