TESTING THE MASS AIRFLOW
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
|poor driving behaviour|
|engine ringing noises|
|engine speed variations|
|power loss under maximum acceleration|
|illumination of the TIL (this is the O/D light)|
These concerns may also be accompanied by the
following Diagnostic Trouble Codes (DTCs)
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:
P1723 Gear 3 incorrect Ratio |
RUN THE MAF TEST
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
2. If any of the above DTCs occur again, check
the Long Term Fuel Trim (LTFT1 and on V6 models, LTFT2) by the following
|Start the engine and bring to operating
|Switch off the airconditioning/climate
control where fitted and select 'N' on the autobox |
|Check 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
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.
TEST WITHOUT OBD LEAD
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.