Posted to Technical Tips Forum on 12/23/2011
Seeing The Whole Picture: The Importance of Loop Status
This Ford van came to me with the typical P0171 and a P0174.
[2001 Ford E-250 Econoline, Scan Data] OK, so it's time
to analyze fuel trims. Whenever I look at trims I always
pull up the Fuel System Status. I see many screen shots
posted here without this pid displayed. Hopefully with this
post I can prove how important it is when looking at fuel
First I need to briefly explain the different fuel system
statuses. We should all know what closed loop and open loop
are so I will not spend any time on them.
Open Loop - Drive, also known as fuel enrichment, occurs
during heavy acceleration and WOT.
On a two bank vehicle Closed Loop - Fault is when one bank
O2 sensor or A/F sensor has a fault. This can be caused by a
bad sensor or by one bank being extremely rich or extremely
lean. The vehicle will then substitute fuel trim corrections
from the "good" bank to the bank it believes has a fault.
Open Loop - Fault is when both upstream oxygen or A/F
sensors on a two bank vehicle have a fault. This could be
caused by two bad upstream O2 sensors or both banks can be
extremely rich or extremely lean. On a single bank vehicle
Open - Loop Fault occurs when the upstream sensor has a
fault. This could be caused by a bad upstream sensor or the
vehicle is extremely rich or extremely lean
OK, enough of the boring stuff, back to the case study.
Luckily this van came in the night before and the engine is
cold. I do a cold start and watch the fuel trims. Here is
what I saw when the vehicle is in Open Loop. [2001 Ford
E-250 Econoline, Scan Data] Shortly after start up the
vehicle goes into closed loop. [2001 Ford E-250
Econoline, Scan Data] Notice that the total trims on both
banks starts to climb towards +40% on both banks.
Not long after the vehicle goes into Closed Loop it then
goes into Closed Loop - Fault. [2001 Ford E-250
Econoline, Scan Data] Notice that the STFTs and LTFTs on
B1 and B2 are exactly the same. If I didn't know the vehicle
was in Closed Loop - Fault I would be pretty confused about
what was going on with the trims right now. At this point
the vehicle starts to surge a little bit.
Not much longer and the vehicle now goes into Open Loop -
Fault. [2001 Ford E-250 Econoline, Scan Data] At this
point the vehicle has a rough idle and is surging because
all of the positive fuel correction has been completely
taken away from it. A quick glance of fuel trims without
looking at fuel system status at this point could take you
down the wrong diagnostic road.
After the vehicle finally gets to full operating temperature
it goes back into closed loop and runs well. [2001 Ford
E-250 Econoline, Engine/Propulsion Scan Data] I then bring
it up to 2500 RPM [2001 Ford E-250 Econoline, Scan
Data] . It's pretty obvious at this point that I have a
vacuum leak. Within 15 minutes of running this vehicle I
know have a pretty good direction before I even pull the dog
Upon further inspections I found that the upper intake
manifold gasket was leaking. I just wanted to make everyone
aware of how important it is to look at the fuel system
status to get the "big picture".
I wish everyone a Merry Christmas and Happy New Year.
Matthew from Illinois
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34 Replies Received