1999 Ford Explorer Intermitten rattling sound

FIREDAWG3996

MEMBER
1999 FORD EXPLORER
6 CYL
2WD
AUTOMATIC
170,000 MILES

My daughters vehicle has this loud rattling sound sometimes when you put it in gear or sometimes when you stop at a traffic light. I tried the listening with a screwdriver and it seems to transmit this sound thru the alternator, I replaced it, not the noise. I tryed both the idler pulley and belt tensioner pulley, no luck there either. It also throws out po171 and po174 codes every once in a while. If I erase the codes they will reappear. I also cleaned the ma sensor. Could this be the "timing belt tensioner" and if so where can I find info on replacing it?

1 Reply

DAVE H

MECHANIC
13,383 POSTS

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
ARTICLE BEGINNING
MASS AIR FLOW (MAF) - SENSOR CONTAMINATION - SERVICE TIP
APPLICATION
1991-99 Explorer
ISSUE
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.
ACTION
Follow the diagnostic procedures described in the following SERVICE TIP. The revised diagnostic procedure is a more accurate means of diagnosing the symptoms.
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.
NOTE:The following procedure may also be used to diagnose vehicles that do not have fuel system/HO2S sensor DTCs.

Symptoms
Lack of Power
Spark Knock/Detonation
Buck/Jerk
Hesitation/Surge on Acceleration
Malfunction Indicator Lamp (MIL) Illuminated DTCs P0171, P0172, P0174, P0175 may be stored in memory
OBDII DTCs
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)