A COMMON PROBLEM: Some of the most common fault codes pertain to the fuel trim.
However, by the time the code is set, a lot of damage may already have occurred. When
the fuel delivery per cylinder is unbalanced, such as a restricted injector or an intake leak,
the ECU sets the trim to satisfy the leanest cylinder with the consequences that all the
other cylinders are seeing an overly rich fuel mixture. In time this will contaminate the O2
sensor, building up carbon, not only in the combustion chamber, but also on the valve
stems, EGR valve, spark plugs, etc.
Since a richer mixture does not affect drive-ability, the customer is not aware of the
problem until many other nasty side effects occur. However, the fuel consumption may be
higher, but very few people keep track since there is not much they can do about it.
HOW FUEL TRIM WORKS:
When a vehicle leaves the factory with new injectors and new vacuum hoses, the fuel
delivery is perfectly equal for all cylinders. The computer is programmed for the ideal Air-
Fuel-Ratio which is considered the base and is identified as ZERO.
After many miles of refueling, it stands to reason that contamination of dirt affects the fuel
system and heat deteriorates vacuum hoses which may cause the fuel trim to move away
from the base setting to compensate for deficiencies. This may be anywhere from vacuum
leaks, restricted injector or leaky injector, etc. This deviation is expressed in %.
WHAT THESE NUMBERS MEAN:
Fuel trim on the positive side indicates that the system detects a lean condition and the
computer is adding fuel. Numbers on the negative side tell us that the computer is
SHORT TERM vs. LONG TERM:
Adding or subtracting fuel is immediately indicated by the Short Term Fuel Trim, expressed
in percent. When the added or subtracted numbers exceed the short term limits, it dumps
the information into the Long Term Fuel Trim storage and Short Term returns back to zero.
It is like taking a waste basket to the dumpster and returning with an empty can.
When the amount of free oxygen has reached a number which allows the O2 sensor to
toggle above and below the Lean/Rich crossover, the computer stops the compensation.
At that time the adding and subtracting will be equal. Whether the offset is positive or
negative, it will stabilize when the deficiency is compensated.
WHAT IS ACCEPTABLE: Any number over ZERO in LONG TERM is a potential problem.
However, when the system is 10% or more away from the original factory setting it is
advisable to investigate. By the time it reaches 30% or more it is considered extreme, to
the point of requiring urgent attention to determine the cause.
IDENTIFYING THE PROBLEM:
When the numbers are negative, the computer is subtracting fuel to compensate for a
rich mixture. The most likely reason is a dripping injector. This will drive the other
cylinders extremely lean, almost to the border-line of misfiring. The result is that on quick
acceleration from a dead stop, the engine may hesitate, stumble and stall.
A leaky injector will be less detrimental at higher speed (2000), since the number of drips
is only 30% of what it would be at idle. To prove this, increase speed to 2000 RPM and
observe the short term percentage number dropping toward zero.
When the numbers are positive, the computer is adding fuel to compensate for a lean
condition, like a restricted injector, or a vacuum leak. To identify the restricted injector the
hookup below allows adding small increments of fuel to any one of the injectors until the
lean injector misfires. The result can be displayed on an OBD-II scanner.
HOW IT WORKS
A steady green light (3) indicates the power input side of the injector. The switching side of
the injector creates an ever so slight blink when the injector is energized, which is less
than 1% of the time. Leave the probe there and proceed with the test. Even if you did not
need help to identify, the green light helps to indicate when you lost contact.
Limit the exploring only to GM and Ford systems until further development is completed.
PROGRAMMING THE SCANNER
After programming the vehicle data, scroll to the misfire data, preferably along with LONG
and SHORT TERM percentage readings.
Tap into negative
side of INJECTOR
Piggy-back FUEL TRIM GENERATOR established
to add FUEL
TEST PROCEDURE FOR LEAN INJECTOR (positive numbers)
Typical restricted or dirty injector
Holding the button (5) on the Fuel Trim Generator adds fuel and drives the computer
leaner to the point where the leanest cylinder eventually misfires. Holding the button
creates a rapid fire and misfire sequence. The misfire count keeps climbing up to a limit of
about 70. Then it stops climbing. This occurs when long-term has accepted the new value
and the O2 sensor resumes its normal routine of bobbing at a regular LEAN/RICH
condition. The number 70 is an arbitrary number which may vary with engine type. The fact
that the count is high is significant for diagnosis. It indicates that only one single injector is
When two injectors are restricted, the accumulated total misfire count of two injectors is the
same number. It takes a certain amount of oxygen created by misfire to offset the
enrichment created by the generator. The conclusion is that a high count indicates that 5 of
the 6 injectors are good and replacing a single injector may be warranted. Let us assume
that we see a low count of 40 on one injector and a 30 count on another. The oxygen
required to offset the enrichment is spread over several cylinders. This becomes significant
information, since the customer can be advised about repair options. When more than one
injector is contaminated, replacing or servicing the entire set should be recommended. A
matched set of rebuilds is a better bargain.
ADDITIONAL OBSERVATIONS: All the above is accomplished by just hooking up to any
one of the injectors, whichever is the most accessible. If the hookup happened to be on the
lean injector, the injector will be identified on the scanner as a misfire but the count is
significantly lower. Moving to a good injector may prove the point.
For lean injector detection, using the FUEL TRIM GENERATOR together with the
scanner as a diagnostic tool is much faster and dead accurate compared to using the
scope as illustrated above. In addition it verifies computer function and O2 sensor all in
one simple test procedure.
TWO BANK APPLICATION: If two O2 sensors are used on two separate exhaust
manifolds, the test procedure needs to be performed for each bank.
CONFIRMING @ STEADY SPEED
This allows time for the computer to
compensate for the lean condition.
LEAN MIXTURE @ SNAPTEST
During the snap-test the computer is
not fast enough to compensate.
TEST PROCEDURE FOR RICH INJECTOR (negative numbers)
Typical leaky or dripping injector
To illustrate the effects of a leaky injector, note the paraded scope pattern below. Cylinder
#3 in firing order is obviously overly rich to the point when the coil stops firing at the end of
the firing time. There is no evidence of a lean trim and fuel is still supplied beyond that
point. Observe there is no “nose” or residual coil energy left. The spark plug is near, but
not yet flooding, since the KV demand still indicates a spark gap.
The result is that the good injectors are driven extremely lean. This is indicated by a rapid
oscillation on the firing time which is evidence of unstable or scattered hydrocarbon,
switching from good conductivity to poor conductivity. There is one exception, which is
almost a perfect scope pattern. Cylinder #3 is under computer control, but has the
advantage of being located next to its rich neighbor to share some of the overflow...
As we saw on the previous page, to capture a lean injector on the scope requires several
steps, like a SNAP-TEST and a FREEZE PATTERN and a SCROLL BACK.
Using a scope to pinpoint a rich injector is much simpler to capture. Since the computer is
not in control (it is a mechanical failure) it can not hide. For details how to verify, see the
video “Through the Eyes of a Scope”
DIAGNOSING THE RICH CYLINDER: What will happen when we add fuel with the FUEL
TRIM GENERATOR? The lean cylinders like it and RPM increases immediately, until the
fuel trim adjusts and brings it back in balance. Eventually cylinder #4 fouls out and
misfires. For a quick cylinder identification using the scan tool, scroll to MISFIRE per
cylinder and apply momentarily a double dose of enrichment with the red button on the
COP-III tool. The leaky injector is the first one to misfire. How long is momentarily? That
depends on how bad the drip was.
All cylinders are
driven lean because
of a dripping injector.
This Ford F.O.
1 5 4 2 6 3 7 8
places cylinder #3
right next to the
dripping #4 cylinder.
an almost perfect
scope pattern on #3.
DYNAMIC AUTO TEST ENGINEERING CORP.
Visit our website: www.datec.us – Contact us at: firstname.lastname@example.org
Phone 269-342-1334 Fax 269-381-4178
A ‘97 Buick came in with a hard starting complaint. The engine was a 3800 vin #1. There
was no drive-ability problem reported and no code set. Just prolonged cranking before the
engine would start. There was nothing unusual noted on the scanner other than that the
Long Term Fuel Trim was at 139.
This number warranted to do a Fuel Trim Test. The FT-Generator was connected to
injector #5 and the first objective was to check O2. The Post Cat. O2 was rather low.
Holding the red button momentarily indicated the following:
2. Good range
on Pre- and
Post Cat O2.
The COP-III was connected to injector #5. Holding
the button did create misfires on cylinder #3, which
eventually maxed out at a count of 35.
Then the probe was moved to the misfiring injector
#3, and enrichment was applied. The misfire count
rose to 134, (see scanner readout to the right).
DIAGNOSIS: The numbers do not make sense.
Injector #3 does not react like a lean misfire. The
probable condition is that all injectors are lean with
the exception of #3. With the computer adding fuel
to satisfy the lean injectors, #3 becomes overly rich.
Adding fuel floods cylinder #3, resulting in misfire
(35). Applying fuel directly to injector #3 really fouls
the plug out (134). NOTE: This is the opposite of
how a lean injector reacts.
NOTE: To speed the test results up or to beat the
long term compensation, a double enrichment dose
can be applied by pushing the red button on the
Since the scan-tool was already hooked up, the
whole test procedure took less than 10 minutes.
TEST RESULTS AFTER THE FIX
Cleaning the injectors fixed the problem. Note the
scanner printout and the O2 printout recorded
immediately after the fix.
The post Cat. O2 will eventually come down and
Long and Short Term F.T. will settle nearer 128.
WHAT DID WE LEARN FROM THIS TEST?
1. We established there was a fuel problem.
2. The Cat. Converter was not functional or
O2 sensor was inoperative.
3. Both O2 sensors were OK in range and in
4. Tested computer control, both STFT and
5. Identified and verified defective injector(s).
6. Determine corrective action required.
WHAT DID WE LEARN AFTER THE REPAIR?
1. Verified that O2 sensors are functional.
2. Verified the Cat. Converter is doing the
3. Verified that fuel delivery was equal for all
4. Fuel trim is back in normal range again.
5. Quality control - passed inspection
"There are not enough Indians in the world to defeat the Seventh Cavalry.." - George Armstrong Custer
- Posts: 1476
Join date: 2012-11-03
Additional HHR Info: 2006 LT 207,000 miles and counting / 2009 2LT 84,000 miles
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