| Check all the air/breather pipes are not perished/split or blocked. Make sure the PCV (Purge Control value) is working (Should be a one way value), and that the Carbon Filter is not clogged. Also the pipes from this PCV valve tends to clog up.
The ECU is pretty good at compensating for loss of air/pressure by over fuelling without any fault codes present.
The Lamda probe (O2 or oqygen sensor) would normally thow up an error code, but not necassarily. Apparently you can check to see if the resistance is varying during a differential in revs. If this is giving a false reading, then it will instruct the ECU to over fuel.
The Idle control valve maybe clogged up. It can be cleaned to make it run a little more efficiently.
Do you have any after market filters. This can effect the ECU/engine management. After market filters for example can cause a thin film of oil on the Air Mass Meter coil, causing the differential resistance to read slightly highter/lower than the current condition, thus sending the wrong value to the ECU and overfuelling.
These are a few idea's to think about...
Good luck.
MAK | 
