89 Volvo

What kind of Volvo is it?

The model will help us narrow the problem down, but my guess is that it is associated with the Mass Air Flow sensor (assuming it is a type that has one.) The MAF sensor in those models is mounted in the intake air ducts right near the air cleaner. The problem could be:

*A really big leak in the air duct between the engine and the MAF sensor. That can be checked by removing the duct on the engine side of the sensor, putting it to your face and blowing in it. If there is resistance, continue on. If there is little or no resistance find the big hole. Either way, wash the silly black black ring off your face. *A bad connection on the sensor. When the idle is running up and down, grab that flat connector and try flexing it up and down (gently but firmly!) and left and right. If the idle suddenly straightens out, remove the connector and clean it with alcohol or similar. It is not advisable to remove and reseat the connector on a whim, since it can introduce intermittents where there was none before. *A bad MAF sensor (they are also called AMMs: Air Mass Meters). They are a couple hundred dollars US new or rebuilt, so wrecking yards are favorite sources of MAFs/AMMs. IIRC there are two varieties - those without adjustments and those with. Also, don't mess with the adjustment. It is much more likely to be correct now than it will be after amateur tweaking.

Be aware - there could be other causes of your symptoms, including fuel delivery. If the first two tests above don't point you to a problem, a fuel pressure check is pretty affordable at a shop. Along that line... if the fuel level is around 1/4 tank add a couple gallons to the tank. If that cures the problem the fuel bellows between the in-tank prepump and the fuel line is torn and sucking air. For now, keep at least 1/2 tank of gas in the car in that case. A proper fix gets fairly expensive.

Mike

A safe one?

Sorry could not resist! :)