Your MAF is most likely shot. A Volvo tech told me a resistance and or voltage test doesn’t definitively tell you if it is working properly. The adjustment screw you are talking about does adjust the fuel mixture, however I have been told that it doesn't make much difference, because the computer does most of that. When I bought a new MAF and installed it I adjusted it. But it was hard to tell if turning it was changing anything or if it was the ECU(computer. I don't think the little bit of crud on the screen is doing anything. What did you use to clean it? CRC makes a good product. You can either buy the MAF cleaner, or their fast dry electronic cleaner for sensitive part. But I have a feeling that your MAF is shot. I had the same problem with my car. In my case someone actually put a MAF from a Saab in my car. Your part # should read 0 280 212 007. If you do get a new one make sure you take all that black foam liner crap out of the box and get a new air box thermostat. If you do decide to get a new MAF I have one in brand new condition for $90 You should see if you can borrow one from somebody to test. But like I said I’m 99% sure that is what you need. If its not that you need an ECU. If you are interested email me V_rizla@hotmail.com I’m not just trying to sell you something I honestly think that is what your problem is. The one I have was in use for 2 weeks, then my car was totaled.