OBD2 Fault Codes -
The advice given in MAF Test is neatly
illustrated by - of all things - a BMW 740i from Germany belonging to Arden,
who contacted us in desperation.
His very nice V8 saloon was coming due for
a TUV, the German equivalent of the MOT. The BMW was running beautifully, but
the MIL (Malfunction Indicator Light) was coming on and Arden could not find
out why. There were no driving concerns, but the MIL light insisted on
illuminating. Arden had the OBD lead and he could turn off the MIL
light, but during the TUV the engineer would check to make sure that the OBD
tests were all complete and if not, the car would fail. But if Arden allowed
all the OBD tests to complete, the MIL light would come on, and that was also
an automatic TUV failure - Catch 22.
Arden consulted two Main Dealers and a BMW
specialist; they ran tests but could not reach a definite conclusion. One
suggested a complete change of valve cover gaskets to cure an inlet leak. Using the BMW version of Alex
lead, Arden read the DTC's causing the MIL light and datalogged the car on a
journey during which the MIL light switched on.
He tried some OBD Forums but none of them
could help him, until he found us on Google. He read the
Clean the MAF advice and did so, but there was only
a marginal improvement.
Arden emailed us asking for help, included a zipped file of
his scans, the first one of which showed everything I needed to see. The very first obvious thing was that the
both Long Term Fuel Trims were clearly in the negative and stayed there all
the time that the engine was idling, and this was confirmed by the DTCs
generated during the scan -
The DTCs P0170 and P0173 show a
Fuel Trim malfunction on both Banks of the engine - a V8 - and this is the first clue. Could the
Dealer have been right and the cam-cover gaskets needed replacing to repair an
air leak? We need to look at the Fuel Trims, and here is the DATA page:
The DATA page while the data log is replaying. I have
highlighted one of the two Long Term Fuel Trims (LTFT) and this is Bank 1 -
the driver's side of the V8. I have clicked on the Stats page and this shows
that LTFT reaches a maximum of -1.563 and a minimum of -8.594 . The average
value for the trim is -7.662. Oh dear - this is a Negative Fuel Trim, and so
is LTFT 2 with much the same result. To confirm that this is at idle we can
use the DASH page to configure a virtual dashboard:
Above, the instruments configured to show the numbers
scrolling through the DATA page. The TPS confirms that the accelerator is not
depressed (the BMW shows 0 for the TPS, whereas the Scorpio typically shows 15
or 16%.) MPH confirms that the car is at rest, the RPM shows as 704, while the
LOAD figure is very low at 4.3%. This all confirms that the engine is at idle
and this is important - the Negative Fuel Trims Syndrome relates to trims at
The two dials bottom centre are configured to show the fuel
trims and both are at -8.6%. Could that Main Dealer be right, that there is an
air leak? Well - no. The Fuel Trims are pressed firmly into negative - that
is, providing less fuel, and to do this it opens the injectors for
shorter and shorter periods (in milliseconds). It is providing less fuel
because the engine management (PCM) believes that the engine is running
rich at idle.
For the dealer's suggestion about an air leak to be true the
Fuel Trims should be positive - increasing the fuelling rate to
compensate for the additional air in the inlet system. There is no sign of
this whatever, and the considerable expense of replacing the valve cover
gaskets would have had no effect whatever - apart from lightening Arden's
wallet and perhaps have shifted the fuel trims even further to negative!
Fortunately, I was able to reply that this was a classic -LTFT
Syndrome. I asked if he had cleaned the MAF thoroughly with a good brake or
Carb cleaner that left no residue, and if he was sure that the element inside was
sparkling clean because the MAF was clearly the culprit.
After a short while Arden mailed us back:
Just thought I'd let you know that the MAF was the problem - it was dirty. I
originally tried a not so inexpensive carb cleaner, but it did not say whether
or not it left a residue. I then finally found one (out of about 10) that did
say no residue, which made a big difference....
Also, I had a
strange problem where the tranny would have a problem downshifting from 2nd to
1st when I slowed down very quickly, which has gotten much better since the
MAF has been cleaned. The MAF took only about 10min to inspect, clean,
reinstall my first time! Many thanks for your great help, and most important
web site! (Please post this somewhere? ) The original error: fuel trim p0173,
then fuel trim p0170...
That OBD2 tool was very informative also :-)
Thank you thank you thank you.
Glad to help, Arden.
Ford Scorpio owners have known for some time that a dirty MAF
could affect autobox operation, and now a BMW owner had discovered this too. But
make sure that the fluid used to clean the MAF is residue free.