There has been a lot of discussion on this and other Ford sites about the Mass Air Flow Sensor (MAF) getting dirty and causing driveability concerns. One particular question that gets asked a lot is whether or not a dirty/faulty MAF can cause or contribute to the dreaded engine knock or "pinging" that we have all heard so much about. Apparantly, Ford determined several years ago that yes, a dirty MAF can indeed be a source of pinging.

This is an excerpt from a 1998 Ford TSB:

Article 98-23-10

Mass Air Flow (MAF)- Sensor Contamination- Service Tip

MASS AIR FLOW (MAF) DISCUSSION

MAF sensors can get contaminated from a variety of sources: dirt, oil, silicon, spider webs, potting compound from the sensor itself, etc. When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air flows.

If vehicle is driven at Wide Open Throttle (WOT) or high loads, the fuel system normally goes open loop rich to provide maximum power. If the MAF sensor is contaminated, the fuel system will actually be lean because of under-estimated air flow. During open loop fuel operation, the vehicle applies Long Term Fuel Trim corrections that have been learned during closed loop operation. These corrections are often lean corrections learned at lower air flows. This combination of under-estimated air flow and lean fuel trim corrections can result in spark knock/detonation and lack of power concerns at WOT and high loads.

One of the indicators for diagnosing this condition is barometric pressure. Barometric pressure (BARO) is inferred by the Powertrain Control Module (PCM) software at part throttle and WOT (there is no actual BARO sensor on MAF-equipped vehicles, except for the 3.8L Supercharged engine). At high air flows, a contaminated MAF sensor will under-estimate air flow coming into the engine, hence the PCM infers that the vehicle is operating at a higher altitude. The BARO reading is stored in Keep Alive Memory (KAM) after it is updated. Other indicators are Long Term Fuel Trim and MAF voltage at idle.

Symptoms:

- Lack of Power

- Spark Knock/Detonation

- Buck/Jerk

- Hesitation/Surge on Acceleration

- Malfunction Indicator Lamp (MIL) Illuminated

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