The 850 and X70 use the vacuum created on the intake manifold from the engine running to open or shut components on the engine. The majority of the lines are made of hard black plastic that may be wrapped in rubber hose for protection, the connection points are almost always rubber elbows. What happens in time is the rubber becomes soft, mushy, and may open up a hole. When a hole is opened, un-metered air is taken into the intake manifold causing a lean condition in the combustion chamber. The oxygen sensor will detect the lean condition (lean on fuel) and the Engine Control Module (ECM) will make adjustments to the air/fuel ratio. Adjustments can only be made so far before the adjustment tables are pegged and a code is set (EFI-232/231) and the check engine light is turned on. The other side of the "Mushy" elbows is that the elbow just gets soft and mushy and closes up, not allowing the vacuum line to do what ever job it's suppose to. When changing your oil it is a good idea to check the "Rubber Elbow" connections on your engine to avoid a possible check engine light later.
The above picture shows the most common failure. It is tucked back out of sight and most people don't even know it's there. To find it on the passenger side of the intake manifold, look down below the radiator hose that goes from your thermostat housing to the radiator. The elbow is on the end of the manifold inboard of the power steering pump. If you take a bungee cord and wrap it around the radiator hose and pull it up on top of the power steering pump you can get a better look at the elbow. You can fit your hand in there and gently feel the elbow to see if it has any holes or it's soft. You can't see it in the picture but there is a green clamp on the hose where it connects to the manifold. I use a 12" pry bar and several long needle nose pliers to gently pry the elbow off the manifold, and then the long needle nose pliers to work the elbow off the plastic line. It is a tricky operation and if it is your first time, the easier way is to move the power steering pump out of the way, which will give you unrestricted access to the elbow.
At the other end of the intake manifold is the throttle body. I have removed the black plastic cover (25mm torx Screw) from throttle linkage to show you what I call the "Vacuum Tower". Depending on the type of engine you have, you will have different hoses connected here; note the three front hook-ups are not used and have plugs on them.
This car is not a turbo so it has a flame trap, with a rubber elbow on the piping that connects it to the throttle body. I have removed the hose held by 7mm hose clamps, So you can see the flame trap.
If you have a turbo then this picture applies to you. If you follow the intake hose down from the air box, you will notice more rubber elbows that need checking. Some of the older turbo models have small medal piping that runs under the spark plug cover for heating, well the piping comes out close to the upper engine mount and at that point you will have rubber elbows that need checking. Once you have replaced the elbows, if a code has been set you need to clear the codes. On 1996 on you would need a scan tool to reset the code and the adaptives, pre-1996 you can use the diagnostic connectors located by the washer fill. To reset the adaptives on pre-1996 you need to disconnect the battery for about 10 minutes. You will need your radio code once you hook the battery back up.
Below is a chart of the various rubber elbows that are found on the Volvo. The drawings are not to scale so look at the size for both ends to try and match up the ones you need. The numbers are the Volvo part numbers.