first, top up the coolant and see if that clears your problem (also, try to find out if your coolant is leaking or not, if its not leaking you may be burning it)
second, check for vaccum leaks, some can be hard to find but you gotta do it.
third, im thinking heat sensor in the MAF as I had that issue.
try the first two as they are much cheaper... good luck
1992 325i / Briliant rot - SOLD @ 301,000km
1998 318is / Arktic silber - SOLD @ 207,000km
2007 335i / Jet Black - CURRENT @ 28,000 km