I recommend rebuilding the DISA with a kit that includes an aluminum flap and a full length screw/shaft that eliminates the chance of the pin that currently holds the flap breaking loose and falling into your intake. Very bad for your valves. Easy way to test if your DISA is working is to pull it, manually close the flap and put your thumb over the small intake port at the vacuum diaphragm. If it stays closed or close to it, it is working. If not, it needs a new diaphragm. There are rebuild kits out there that include this diaphragm. This shows how to test the DISA on an E46: https://www.youtube.com/watch?v=pg6Jb3qEYvM It's essentially the same valve as the E53. Function is the same. Test demo starts at 1:46.