I have a 2000 X4.4i (77000 miles) and I was getting

P0170 Fuel Trim Malfunction Bank 1
P0173 Fuel Trim Malfunction Bank 2

I tried cleaning the MAF, I cleared the codes, but the codes returned a few miles later. Finally gave in and paid $240 for the MAF and replaced it. All is well - no codes. By the way, prior to these codes, I had already replaced the Oil separator top hoses. the other hose, behind engine block was ok.

Just in case, check all the plastic connections (hoses) between air filter box and throtle boby - some members have posted problems with cracked plastic hoses.


Hope this helps