Hi!

My 2.0T 1999 Volvo is running lean and I知 suspecting the problem is with MAF. I知 getting reading about 4g/s idling. What is the mechanism of this as when I take off the EVAP hose to introduce a air leak, the MAF value rises to 8g/s? I would think less air would go through the MAF.

 

not sure where the evap hose is in relationship to the MAF but the way I think of it is that the engine is an air pump and its sucking air in through the throttle body and the air going into the throttle body flows through the MAF. The MAF has a grid of wires that are either heated or not. When the air flows over the wire it cools the heated wires changing the resistance and when that's compared to the non-heated wires the computer can calculate the airflow. So if you introduce an air leak between the MAF and the engine intake ports your MAF will give the ECU an incorrect metering. Now say you introduce an air leak say between the turbo and the MAF (ie a torn intake hose or vacuum line) what gives? Its not the MAF at this point, there's other sensors like the MAP (your car may have two MAP sensors! - I know my 2000 S40 does) which is measuring air pressure in the intake so that could be where a false reading to the ECU comes from. Then there is the O2 sensor on the exhaust side which also contributes to fuel trim management.