This may help, I had typed this up for someone over at MD, and thought I'd just copy past it instead of typing it all over again.
The MAF Sensor uses hot wires inserted in a known cross sectional area to measure airflow. The MAF Sensor and the IAT Sensor output signals to the PCM determine the injector pulse time during Open Loop Condition. The MAF output signal is dependant on the resistance in the wire due to the wire temperature. The temperature is a function of the air velocity, as the faster the velocity the lower (cooler) the wire temperature. So, as the velocity of the air increases (acceleration) the wire cools and sends a signal to the PCM to lengthen the pulse time of the injectors to deliver more fuel in order to maintain the desired A/F ratio of 14.7:1.
In front of the heated wire is a screen. Many people are suggesting that improved airflow will be gained by cutting the screen out of the MAF. It is not recommend that this be done. Most people mistake this screens purpose as preventing debris from hitting the hot wires in the MAF (that is what the air filter is for). Actually the screen is a bank of straightening vanes used to straighten the airflow before passing by the hot wires. Eddies and vortices (turbulence) in the MAF will cause inaccurate readings. Straightening vanes are used in most mass flow measurement situations where 10 diameters of straight upstream conduit are not available. The intake does not meet this requirement.
An accurate MAF sensor is critical for the engine's fuel/air ratio adjustments. So, what happens when the screen is removed? The flow (mass) is the same, BUT without the screen the velocity is lower. Lower velocity means a hotter wire temperature which equates to a LOWER flow! So the PCM adjusts the injectors to provide LESS fuel. Since the HO2 sensors do not provide input when the MAF is online the PCM does not adjust for the LEAN condition, until it goes back into Closed Loop Condition.
