Hello and thanks for using JustAnswer!
I am Derek, a master certified Volvo technician i will be happy to assist you with your vehicle.
What is the model of your Volvo?
Turbo or non turbo?
Does it smoke out of the exhaust?
Thank you! Do you know the fault code that is set in the engine control module?
Does the engine seem to run ok?
any over heating? White smoke from the exhaust could indicate a failed headgasket.
Blue or black smoke is either fuel or oil related.
If it is using oil, and dark colored smoke from the exhaust i would recommend starting with replacing the flame trap box.
The flame trap box, or PCV box is known for a internal blockage and will cause the problem you are experiencing.
A failed egr valve will have a very rough idle, and run ok at higher speeds. But a failed EGR will not cause smoke from the exhaust.
Piston rings are possible but not as common.
The most common cause for smoke is the flame trap box. Also with a pcv system problem the engine will seem to run ok.
You can remove the spark plugs and inspect them if you suspect the rings are the problem. Also a compression check can be done. Low compression or worn out spark plugs can cause oil to bypass the rings and burn in the engine.
That would be a very good place to start. Also inspect the spark plugs for being oil fouled. Sometimes the pcv passages can become plugged.
Let me know what you find