By Bob Pattengale | September 2006
No
matter what the driveability issue happens to be, checking the PCM�s
fuel trim decisions first can get you pointed in the right direction,
and may end up cutting your diagnostic time in half.
The year is
2006, and for those who may have overlooked it, this is the 10th
anniversary of On-Board Diagnostics II (OBD II). I believe it's cause
for celebration. Here's an example of how it used to be: I was recently
called to help a shop with a 1992 Subaru. In order to retrieve the
diagnostic trouble codes (DTCs), I had to remove the driver's kick
panel, visit a vehicle repair information source for the instructions
on how to jumper the diagnostic connector, then count the flashes of
the malfunction indicator light (MIL). The final step was looking up
the DTC description. Total time from start to finish was approximately
15 minutes. If this had been an OBD II vehicle, I would have had the
information in under 30 seconds. The standardization associated with
OBD II, which gives us easy access to fuel trim data, has really
simplified the diagnostic process.
What is fuel trim?
Fuel trim is a window that allows you to see what the computer is doing
to control fuel delivery and determine how the PCM's adaptive strategy
is operating.
Why was fuel trim created? In order for
vehicle manufacturers to comply with EPA emissions regulations,
catalytic converters were added to reduce tailpipe emissions. Catalytic
converters need a stoichiometric air/fuel ratio of approximately 14.7:1
to obtain the greatest emissions reductions. Vehicle engineers designed
closed-loop engine control systems to maintain that ratio, adjusting
injector pulse width based on information from the oxygen sensor and
other inputs. Short-term fuel trim (STFT) and long-term fuel trim
(LTFT) are expressed as a percentage, and the ideal range should be
within 65%.
Positive fuel trim percentages indicate
that the powertrain control module (PCM) is attempting to richen the
fuel mixture, to compensate for a perceived lean condition. Negative
fuel trim percentages indicate the PCM is attempting to lean out the
fuel mixture, to compensate for a perceived rich condition. STFT and
LTFT percentages are the adjustments made by the PCM to maintain the
14.7:1 ratio.
No matter what the driveability issue
happens to be, the fuel trim window should be used first to check the
STFT and LTFT parameters.
There are two basic fuel
control systems used on most vehicles: speed density systems, which use
rpm, manifold absolute pressure (MAP) and barometric pressure (BARO) to
calculate engine load, and mass airflow systems, which use the mass
airflow sensor (MAF) and rpm to calculate engine load. In both cases,
the PCM begins with a standard injector pulse width calculation, based
on various inputs and internal fuel cell tables.
The
equation used by early Chrysler speed density OBD II vehicles to
establish initial pulse width is: Injector Pulse Width 5 (RPM 3
MAP/BARO) 3 TPS 3 ECT 3 IAT 3 Battery Volts 3 O2 (Short Term x Long
Term). Once the vehicle is running and the engine control system enters
closed-loop, the PCM relies primarily on feedback from the oxygen
sensor to determine if the stoichiometric air/fuel ratio is being
maintained.
Think of closed-loop operation as a
Sense-Decide-React sequence. The Closed Loop System Operation sequence
in Fig. 1 on page 66 provides an explanation of the Sense-Decide-React
process. The PCM determines the base injector pulse width as described
above. Once the system enters closed-loop, the Sense phase begins, and
is handled by the oxygen sensor. In the Decide phase, the PCM uses the
oxygen sensor data to determine if the proper 14.7:1 air/fuel ratio is
being maintained. If the ratio is correct, the PCM decides that no
change should be made to the injector pulse width. In this scenario,
the React phase maintains the same injector pulse width. However, if
the air/fuel ratio is 16.1:1 (lean) during the Sense phase, the PCM
makes the decision to increase the injector pulse width to correct the
lean air/fuel ratio condition. In the React phase, the PCM commands the
fuel injector to stay open longer. The Sense-Decide-React sequence
continues throughout closed-loop operation, maintaining the proper
air/fuel ratio.
During closed-loop operation, the PCM
reports changes in fuel trim calculations via the OBD II generic data
parameters short-term and long-term fuel trim. STFT for most vehicles
will normally sweep rapidly in response to the oxygen sensor. In many
cases, if you graph Bank 1 STFT and B1S1 O2 sensor, you'll see the
oxygen sensor go rich and STFT go lean to adjust the air/fuel ratio.
The oxygen sensor will then go lean and STFT will go rich.
LTFT
for most vehicles will remain more stable, adjusting over a longer
period of time. On some vehicles, if STFT has reached the specified
limit, LTFT will change in a few seconds. On other vehicles it may take
15 to 20 seconds before a change occurs. The LTFT calculation is
normally kept in memory, so the PCM is ready to use the last known
injector pulse width following a restart. STFT will normally begin at
0% and adjust to the current conditions. Both STFT and LTFT will
normally reset when all trouble codes are cleared.
To
better understand how fuel trim is used to maintain the proper air/fuel
ratio, look at the set of fuel injectors in Fig. 2, which are being
tested prior to cleaning/rebuilding. The injectors were removed from a
2000 Honda Odyssey that had idle quality and fuel trim problems, with
related DTCs. You can see some differences in the spray patterns and
volume. Injectors 1, 3 and 5 look very similar in spray and volume.
Injector 2 seems to spray a little less volume. Injectors 4 and 6 have
even less volume, and the spray patterns are not good.
Fig.
3 shows the total volume the injectors flowed in 30 seconds at 40 psi.
The actual injector volume seems related to the spray patterns from
Fig. 2, but knowing exactly how much flow occurred provides a better
picture. Let's take a closer look at how injector volume and fuel trim
relate to one another.
Injectors 1, 3 and 5 are very
close in flow volume�approximately 61 to 64mL. For discussion purposes,
we'll use 64mL as a baseline for 100% correct injector volume, or a
14.7:1 stoichiometric air/fuel ratio. One thing I noticed right
away�and this turned out to be just a coincidence�was that the
even-numbered injectors all had flow issues. On this vehicle, the
affected injectors are actually on different banks (see the Firing
Order table in Fig. 3). If all the even-numbered injectors were on one
bank, it might indicate possible contamination or fuel flow
restrictions in the fuel rail. Also, we do not have bank-to-bank fuel
control for this particular vehicle, so the LTFT will be an average of
all injectors.
If we compare the best injector (No. 1)
to the worst (No. 4), the difference indicates that approximately 30%
less fuel is being delivered to cylinder 4. If we look at the
closed-loop process for cylinder 4, the oxygen sensor would have
reported to the PCM that the air/fuel ratio was excessively lean. The
PCM would have commanded an increase in injector pulse width the next
time the injector supplied fuel to cylinder 4.
The
ultimate goal of the PCM is to return cylinder 4 to a 14.7:1 air/fuel
ratio. The STFT parameter in the OBD II generic scan tool would have
reported STFT at approximately 130%. To complete the cycle, the oxygen
sensor reports the results of the pulse width increase back to the PCM.
If the air/fuel ratio is now correct, no further adjustments are
required. Over the next few cycles, STFT and injector pulse width will
stabilize. The next step is for the PCM to make a permanent LTFT
correction, if required.
If this were a one-cylinder
engine, LTFT would eventually report +30% and STFT would return to 0%.
In some cases, the PCM might limit LTFT to a specific maximum or
minimum value. For example, if the maximum LTFT adjustment is +25% and
the total fuel trim adjustment is +30%, then LTFT will report +25% and
STFT will report +5%, for a total fuel trim value of +30%. The LTFT
calculation is kept in memory on most vehicles, so the PCM does not
need to relearn the fuel trim calculation the next time the vehicle is
started.
The firing order for this engine is
1-4-2-5-3-6. Let's look at how the injector flow issues will affect the
balance of the engine. Cylinder 1 is normal, cylinder 4 is lean,
cylinder 2 is lean, cylinder 5 is normal, cylinder 3 is normal and
cylinder 6 is lean. As you can see, the fuel injector issue might
create a rough idle condition. If only one injector was failing, the
PCM should be able to stabilize fuel trim and control idle speed within
an acceptable range. However, with three cylinders causing problems, it
would be very difficult to maintain a good balance.
How
will the PCM average out LTFT? If this engine had bank-to-bank fuel
control, we might expect Bank 1 LTFT to be close to 0%, and if we
averaged Bank 2 LTFT the adjustment might be approximately +20%.
However, this particular Honda does not have bank-to-bank fuel control,
so the average LTFT will most likely be approximately +11% and STFT
will be constantly changing from 0% to +20%. Various vehicle
manufacturers employ different methods to make these adjustments; the
important thing is to observe the differences among cylinders when
diagnosing fuel trim issues.
Based on the data from the
fuel injectors, what DTC do you think should be present? I would have
assumed a P0171 (Fuel System Too Lean). Actually, the codes present
were P1491 (EGR Valve Lift Insufficient) and P0172 (Fuel System Too
Rich). The EGR DTC is listed as a possible cause for a P0172 and should
be corrected first. The EGR system was checked and cleaned, but the
P0172 returned. What should the next step be?
The next
step is to determine whether the condition exists over more than one
operating range. Fuel trim should be checked at idle, 1500 rpm and 2500
rpm. In this case, STFT at idle was approximately 223% and LTFT was
24%, for a total fuel trim calculation of 227%. No matter how long the
vehicle idled, LTFT never went above 24%. In this case, STFT carried a
greater weight than LTFT. STFT and LTFT at 1500 rpm and 2500 rpm each
were approximately 3%, which is within normal operating range. Our
diagnosis will need to focus on a condition that occurs only at idle.
After
checking all the items that might cause a rich air/fuel ratio
condition, the only remaining possibility had to be the fuel injectors.
I performed an injector balance test (described below) using fuel trim
data from a graphing OBD II generic scan tool to confirm the injector
issue. Fig. 4 on page 70 shows a close-up of injector No. 4 leaking
fuel under normal fuel pressure while sitting in the test stand.
Remember, an injector tip is subject to intake manifold vacuum and a
leaking injector might be worse under vacuum conditions. A new set of
injectors fixed the idle quality and fuel trim DTC.
If
you suspect a problem with the fuel injectors, use STFT or LTFT to
check proper operation. Fig. 5 is a graph of LTFT from a 2000 Saturn
with a cylinder misfire. The baseline is 210% LTFT. So the PCM is
decreasing injector pulse width to compensate for a slightly rich
condition.
Performing an injector balance test is
simple, as long as you can gain access to the fuel injector connectors.
Unplug one injector at a time and wait until the maximum LTFT change is
reached. On some vehicles you'll use STFT for this test, or a
combination of both STFT and LTFT. With one fuel injector unplugged,
the oxygen sensor will see a lean condition and the PCM will compensate
by increasing the pulse width of the functioning injectors to reach
stoichiometry. The results of this particular test, with injector No. 1
unplugged the LTFT change is approximately +14%, injector 2 +10%,
injector 3 +17% and injector 4 +16%. Injectors 3 and 4 contribute a
greater volume of fuel than injectors 1 and 2. We know this because the
amount of fuel trim increase is greater with these injectors unplugged.
Injector 2 is the cause for concern; with injector 2 unplugged, the
remaining injectors need to supply only +10% total. This injector may
have a slight leak that's causing the negative fuel trims. A new set of
injectors fixed this vehicle. Unfortunately, I was unable to test the
old injectors.
One of the more difficult DTCs to
diagnosis is P0171 (Fuel System Too Lean). The first item to get
replaced is often the oxygen sensor, but most of the time this will not
fix the problem. A dirty MAF sensor can cause this issue, but that
diagnosis can be tricky. Fig. 6 shows fuel trim and volumetric
efficiency charts related to the MAF sensor. This data was captured
from a 2000 Pontiac GrandAm. The fuel trim chart on the left shows
negative fuel trims at idle and positive fuel trims at cruising speeds.
This is typical of a dirty MAF sensor. The MAF sensor overestimates
airflow at idle, which causes the negative fuel trim values. It then
underestimates airflow at cruising speeds, which accounts for the
positive fuel trim values.
The volumetric efficiency
was checked to confirm the diagnosis. The red graph represents
calculated VE based on engine size and engine speed. The yellow graph
represents the actual grams per second recorded during the test. As you
can see, the MAF sensor is overestimating at idle and underestimating
at cruising speed. A new MAF sensor will correct this fuel trim issue.
VW
and Audi use a slightly different fuel trim strategy from other
manufacturers. Fig. 7 is a screen capture from the Vetronix/Bosch
KTS-650. Additive Mixture Correction 1 is STFT in OBD II generic and
will change only during idle operation. Multiplicative Mixture
Correction 1 is LTFT in OBD II generic and will change only during
cruising speeds.
No matter what the driveability issue
happens to be, begin with STFT and LTFT. The PCM will usually point you
in the right direction. Once you know what the PCM is thinking, in many
cases you can cut your diagnostic time in half. Finally, don't forget
to check for specific fuel trim diagnostic suggestions provided by the
vehicle manufacturer.
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