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Testing the MAF

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The MAF is a sensitive and accurate device and is relied on to a great extent for the fuelling and Load calculations made by the EECV Powertrain Control Module (PCM).  The Comprehensive Component Monitor (CCM) programming checks the MAF for high and low values (P0100 to P0104), and also checks the MAF input for rationality by comparing it with the Throttle Position (TP) sensor data, (P1121) but the latest advice is that the programming may not be sufficiently sensitive to detect a corroded or failing MAF sensor. As the MAF fails it may read too high;  this leads to excessively lean fuel mixture and, in automatics, incorrect gear selection.

The function of the MAF is described in Module and that it needs regular cleaning is in Clean MAF. However, sometimes cleaning the MAF does not produce the right result. On the Scorpio automatic, a dirty or failed MAF can produce the loss of 4th gear engagement, incorrect ratio errors and TCC errors, often associated with brief flashes from the TIL light. But if after cleaning it, how do we decide if the MAF is faulty when the faults persist?  One owner of a 16v (Simmo) reports that total failure of the MAF sensor made the engine race, then stall, so if you get this symptom the MAF is the first thing you should check.

Ford advice details some or all of the following concerns :

bulletpoor driving behaviour
bulletengine shudder
bulletengine ringing noises
bulletengine speed variations
bulletpower loss under maximum acceleration
bulletillumination of the TIL (this is the O/D light)

These concerns may also be accompanied by the following Diagnostic Trouble Codes (DTCs)

bulletP0171     Bank 1 Fuel System too Lean
bulletP0174     Bank 2 Fuel System too Lean (V6 models only)
bulletP0172     Bank 1 Fuel System too Rich
bulletP0175     Bank 2 Fuel System too Rich (V6 models only)
bullet P1130     Lack Of HO2S11 Switch - Adaptive Fuel At Limit
bullet P1131     Lack Of HO2S11 Switch - Sensor Indicates Lean 
bulletP1132     Lack Of HO2S11 Switch - Sensor Indicates Rich
bullet P1150     Lack Of HO2S21 Switch - Adaptive Fuel At Limit (V6 only)
bullet P1151     Lack Of HO2S21 Switch - Sensor Indicates Lean (V6 only)
bullet P1152     Lack Of HO2S21 Switch - Sensor Indicates Rich (V6 only)

 NOTE: Some vehicles may show these DTCs with no driveability concerns shown above - but even so this check should be carried out to ensure that the MAF is not the cause of the problem. Of course, these DTCs can also be caused by other problems too - in this case they are being used as an indication.

Scorpio cars with a dirty MAF element have also shown another error: 

bullet P1723    Gear 3 incorrect Ratio 

Whether or not there are any driveability concerns shown above, if one of the above DTCs are found then carry out the following checks. This is based on the latest experience. You need access to an OBD lead.

1. Clean the MAF. Check the element for any sign of corrosion. Reinstall, clear the DTCs and test run the vehicle for a complete OBD cycle.

2. If any of the above DTCs occur again, check the Long Term Fuel Trim (LTFT1 and on V6 models, LTFT2) by the following procedure:

bulletStart the engine and bring to operating temperature
bulletSwitch  off the airconditioning/climate control where fitted and select 'N' on the autobox 
bulletCheck the Long Term Fuel Trim(s) using the Enhanced page on DATA.


Here, the Enhanced page showing the LTFT on PID# 1156 and 1157. These are the locations for a 24V - DOHC engines will differ and will only have LTFT1. You can see that they are both in positive values, and each is different, which is correct.


If one bank is negative but the other is not then the MAF is not the cause


If the measured negative value of LTFT1 (and/or LTFT2) is below -12% then the mixture has been made leaner, perhaps because of the faulty operation of the MAF (too rich at idle).


Now read off the MAF voltage reading. (See pix - MAFV is at PID# 1177 and is returning 0.234.  NOTE - this engine is not at Idle.  Correct reading should be between around 0.6-7 Volts )


If the MAFV reading is higher than 1 volt at idle then the MAF is faulty and must be changed.

With the Scorpio there are often no driveability concerns which point to a problem, but if your Scorpio has a fault that can be detected - perhaps a faulty gearchange and a flashing TIL light - one more test can determine if the MAF is to blame - this is by invoking LOS. 

Limited Operation Strategy is invoked when the PCM cannot obtain enough data to complete its calculations, or where a faulty sensor puts the autobox at risk of damage. Rather than simply stopping the engine and stranding the owner, the PCM provides for an ignition spark at 10 BTDC and minimum fuel for combustion. Torque and RPM is strictly limited and LOS is obvious to the driver.

In the case of a suspect MAF it is very easy to remove its influence and check the vehicle operation. Simply disconnect the MAF multiplug and then test drive the vehicle. The fuelling is now solely by RPM/ECT and if the noticed fault does not reoccur then the MAF is the culprit.  NOTE: the fuelling is now in Open Loop - full LOS may not be experienced if other sensors are providing satisfactory data.

If the problem continues after the MAF is disconnected then it is not the cause of the problem. If the Long Term Fuel Trims (LTFT) are showing consistent negative trim at idle and the car has difficulty starting when hot then consider the Fuel Pressure Regulator - it may be overpressuring the fuel rail. If the LTFTs are consistently in positive trim during idle and the engine is difficult to start while cold, then suspect that the fuel pressure regulator is under-pressurising the fuel rail.

Without an OBD lead the LTFT readings are not available to the owner, unless he pays a local OBD-equipped garage to obtain them. Instead, he could check the MAFV reading at the plug with a multimeter and still do the LOS test.




Copyright 2004