Quote Originally Posted by Venom
what car? Maybe a small vacuum leak? Also worth while cleaning the sensors with electrical contact cleaner.
It's an e91 320d with the M47 engine. Yeah I suppose cleaning the MAF sensor can't do any harm.

Quote Originally Posted by Roberto
Refit the old part and see if problem disappears?

Reset the ECU (disconnect battery) to reset the "learn" function?

Check the air intake hoses you moved, they may have been pulled out of place slightly. Or you may have ripped one of the rubbers during installation.
I'll have to go looking in the bin for the old filter. A new one will set me back 80 quid, so that might be the best option if it will fix it.
I wouldn't have thought I'd have to disconnect the battery when changing an air filter, but I suppose it can't do any harm.

Has anybody else ever seen performance gain when using a K&N filter with a diesel Beemer engine? I had one in my old petrol e39 and there was no difference at all.