I have a 2005 Volvo s40 T5, the engine was making a weird noise, so I looked around and I found out that when I took off the oil fill cap or the dip stick the noise went away, also if I put my hand over the oil fill cap it would suck my hand to the opening, so I looked around here and found that the problem was the Oil Trap "Oil filter housing" here is the post that I found this info and someone posted some good pictures on page 2 - 4
Solution: go to your Volvo dealer and buy the oil trap it costs about 120.00 usd, get two hose clamps
oil, the oil trap should come with a oil filter
Total time on the ground less than 2 hours
if you are tech with all the proper tools, probable less than a hour
to replace the oil trap:
1: disconnect the air tube that crosses across the valve cover there are a few air hoses connected to this tube so be caution, then remove the plastic cover on the valve cover it should be a t27 hex bit.
2: on the air filter box, remove the air filter, then to the left of the air filter is a breather tubs, remove them so you can gain access, next it might require a little force, pull the air box up, it is held in by rubber grommets there are about three, once you get it up a little, on the right side of the air box there is a plastic cover remove it and you will see two plugs for wires, you will need to push in a tab and lift the right release tabs out and the two plugs will release, next on the back of the air box there is a hex screw holding the wires you just released to the box, remove this and pull the wires away form the box make sure there is no other tubes or wires connected to the box and wiggle the air box as far as you can to the drivers side of the car, note you don’t need the air box out of the car, and you can leave the radiator fans, and the intake manifold still inside the car, no need to remove them. so you now have the air box to the right Good.
3: on the oil trap you will see a plug, remove this from the oil trap, then remove the oil dip stick bolt, should be 10mm socket, also there are two more 10mm bolts that hold on some air tubes do the bottom one in the next step, next remove the one top bolt for the oil trap it is on the top right side of the oil trap, next on the air tube that goes from the oil trap to the valve cover you should remove both hose clamps and remove if you can you only will need one side free to get the oil trap out.
4: under the front of the car, remove the plastic splash guard, should be 7 hex screws three in the front, two on the side, and two in the back, once removed you will be able to see the bottom of the oil trap where there are three 10mm bolts holding it on, before removing the bolts remove the bottom bolt of the air tubes from step 3, now remove the three bolts on the bottom of the oil trap, the trap will leak some oil, go back to the top of the car and gently pull the oil trap down to release a hose hooked to the intake, and pull the oil trap out a little, there is a group of wires that are connected to the oil trap you will need to free them it is best if you use long needle nose pliers, once that is removed you can then gently pull the oil trap out between the air box, intake, and radiator, it might be a little tight but it will come out,
5: so you have the new oil trap put the short hose on the oil trap the one with the 90 degree bend, and put the long hose that goes to the valve cover on at this point, you might want to put on a hose clamp at this time, next the screws that you removed to get out the dip stick and the air hoses, you might want to screw though into the oil trap before you put it in so you can thread the holes for the self taping bolts, then remove the bolts before putting the oil trap in, done good
6: put the oil trap in, first you will want to clean the contact area mainly where the three holes in the engine block are you want a good clean contact for the oil seals on the oil trap, all clean now it is time to put the oil trap in, make sure you don’t get any wires or hoses between the oil trap and the engine block, once you have made contact put the oil trap bolt in the top right corner of the oil trap, just make it finger tight, next put in the three bottom bolts also finger tight now tighten them hand tight do not over tighten, if not sure call your Volvo dealer to get the tighten specs, now the rest is just putting every thing back together and do the rest of your oil change,
7: all back together, and oil is filled and oil drain plug is tight, good, time to start the car and turn it off immediately, and do it two more times this is to make sure you have oil pressure, after the third time leave the car running, make sure your oil light is not on in your cluster panel, next with the car running open the oil fill cap and see if it is still sucking air, it will have a little but it should not create a suction to your hand,