I've been trying to figure this out. It runs beautifully but continues to pop up the P2178 code "System too rich off idle Bank 1" After clearing the codes I've replaced both upstream and downstream O2 sensors, Coolant temperature sensor, cleaned MAF sensor and still no luck. I have a Snap On scanner and have a snapshot of the event that causes the code to be thrown....
ENGINE SPEED: 1560
ABSOLUTE THROTTLE POSITION(%): 19.2
FUEL SYSTEM 1: CLOSED LOOP
FUEL SYSTEM 2: NOT USED
INTAKE AIR TEMPERATURE (F): 91
ENGINE COOLANT TEMPERATURE (F): 185
AIR FLOW RATE (g/s): 20.00
IGNITION TIMING ADVANCE (degrees): 33.0
SHORT TERM FUEL TRIM BANK 1(%): -4.6
LONG TERM FUEL TRIM BANK 1(%): -18.7
FUEL RAIL PRESSURE (psi): 57.8
VEHICLE SPEED (mph): 49
CALCULATED LOAD VALUE (%): 38.4
Any ideas would be extremely appreciated

 

I see a big problem with the long term fuel trim. The computer is reducing the amount of fuel by 18.7% from its fuel map (original program) over time. I would start there.

I have had two Volvos recently that set codes related to fuel trim. I fixed both with a new air mass sensor. Those air mass sensors can go out of calibration and in your case it appears that the computer thinks that there is more air coming into the engine than there really is.

Try replacing it with an inexpensive knock off and see if that helps.

 

I have the same problem with our 2011 S40 T5. Were you able to resolve the problem such as by replacing the MAF sensor?