Volvo 240DL: it trembles when I start it...

Does anyone know how i can adjust a 1994 940gl idle speed. I have set everything to spec, including the base idle and microswitch, and the throttle body and idle stepper motor have been cleaned, but it still idles too slow. THe motor still drives ok and doesnt lack power or blow smoke.

It never stalls or has any hunting or unusual behaviour, and the idle will adjust when load is placed (such as a/c) but it simply idles too SLOW. And therefore the motor vibrates, making an otherwise pleasant car to drive seem quite primitive. I have tried simply re adjusting the throttle spindle stop out of spec - to make the minimum idle speed higher, but that had the effect of making the idle speed erratic.

I have been led to beleive that the idle is not adjustable, and set by the bosch engine controller at the factory to a very low 650 to 750 rpm. About half the time it seems to idle ok (not as smooth as i would like but i wouldn't be complaining now if it was like that all the time), the rest of the time it is too slow and make everything in the car vibrate. I cant put my finger on any particular cause (like after a long drive or idling for too long), it just seems to drop off by about 50 rpm and vibrates.

Also, If there isnt any particular fix or reason for this problem, does any one know of any way to trick the computer to increase the idle speed or to activate the stepper motor just a little bit more - therefore increasing the idle speed.

Ps. I think it is interesting that my 85 240gl with the same motor but older (adjustable idle speed) feul injection system has an idle speed of

950rpm. It makes me think that the lower speed on the newer car is too low & set that way for feul or emissions reduction