When the MAF sensor is disconnected the ECU will go to a default system and the ECU relies on other sensors to determine the air-fuel mixture. These sensors include the oxygen (O2) sensor, manifold absolute pressure (MAP) sensor, and air/fuel ratio (A/F) sensor. These sensors provide feedback to the ECU, allowing it to adjust the fueling and timing accordingly. This can result in a rich fuel mixture which feels like the engine is running okay.

We would suggest that the MAF is either dirty or faulty and so will need cleaning or replacing. Note that leaving the MAF disconnected for a long period will increase fuel consumption, bad idling, poor performance and could cause long term damage.