When the engine is cold, the ECU reverts to open loop - that is, a basic preset configuration. Once engine temperature is in the operating range, the ECU switches to closed loop where input is taken from sensors and the fueling lookup tables are consulted by the ECU for fine adjustments.

I don't know if a bad MAF can switch the ECU into open loop, but maybe so. If the MAF is bad, you should have a code stored in the ECU..... but since this MAF is a multi-function sensor (barometric pressure, air temp, air flow, etc.) some MAF functions may not be code reporting. Poll the ECU for codes first.

Frank