LEAN DRIVEABILITY SYMPTOMS: MAF SENSOR CONTAMINATION
TECHNICAL SERVICE BULLETIN
Reference Number(s): 98-23-10, Date of Issue: November 23, 1998
Related Ref Number(s): 98-23-10
MASS AIR FLOW (MAF) - SENSOR CONTAMINATION - SERVICE TIP
Model(s): Ford: 1990-97 Thunderbird
1990-99 Mustang, Taurus SHO
1991-99 Crown Victoria, Escort,
Lincoln-Mercury: 1990-97 Cougar
1991-99 Continental, Grand Marquis,
Sable, Town Car, Tracer
1993-98 Mark VIII
Light Truck: 1990 Bronco II
1994-97 F Super Duty, F-250 HD
1994-99 Econoline, F-150. F-250 LD,
1997-99 Expedition, Mountaineer
1999 F-250 HD,
Super Duty F Series
Bulletin No: 98-23-10
Date: November 23, 1998
This TSB article is a diagnostic procedure to address vehicles that exhibit lean driveability symptoms and may or may not have any Diagnostic Trouble Codes (DTCs) stored in memory.
Follow the diagnostic procedures described in the following SERVICE TIP. The revised diagnostic procedure is a more accurate means of diagnosing the symptoms.
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.
NOTE:The following procedure may also be used to diagnose vehicles that do not have fuel system/HO2S sensor DTCs.
Lack of Power
Hesitation/Surge on Acceleration
Malfunction Indicator Lamp (MIL) Illuminated DTCs P0171, P0172, P0174, P0175 may be stored in memory
P0171, P0174 (Fuel system lean, Bank 1 or 2)
P0172, P0175 (Fuel system rich, Bank 1 or 2)
P1130, P1131, P1132, (HO2S11 lack of switching, Bank 1)
P1150, P1151, P1152, (HO2S21 lack of switching, Bank 2)
Friday, December 5th, 2008 AT 12:30 PM