OK I've got a problem that makes me starching my head... The car in question is a 89 S5 TII with stock drivetrain and ECU. Car runs and drives well, but when it's fully warmed up and idling it will *try* to idle at ~1000 rpm, mostly smooth with a gentle and occasional *cough*.
Now is the weird part: when I manually force the engine to rev lower than 750(i.e. in gear and half-bite the clutch with engaged e-brake), it will ALWAYS goes back to 750 and idle perfectly smooth there and stays there, but if I rev the engine up a bit it will go back to the previous 1000 rpm idle mode.
Both idle modes are stable by themselves..........O_o

additionally, the 750 idle speed could be slightly adjusted via BAC valve, but the 1000 seems inert to BAC adjustments.
and my narrow TPS is actually dead on 1K. I don't have a Mazda dual light tester though...

Any thoughts?
Thanks...

 

throttle cable is taught, throttle body shaft is lightly binding or your thermowax is out of adjustment or sticking open slightly.

generally this is a thermowax issue but could simply be someone adjusted the throttle cable too tight. the throttle shafts sticking is uncommon.