Fuel Trims Explained
I bought an Ultra Gauge that shows my fuel trim both short and long term. This post is all the information I could find on furl trims and is not currently edited for readability.
Chevy Cobalt's have one O2 Bank Sensor (for the V4 four cylinder engines), other vehicles (V6's or V8's) have 2 banks of oxygen sensors and trims associated with that. So in the picture below you see
Srt tr1 and Lng tr1 Stands for
Short Term Fuel Trim (-O2 Bank 1-) &
Long Term Fuel Trim (-O2 Bank 1-) I think
Anyway, a alarm with your OBDII sensor onboard computer will not trigger until things get to around 25% incorrect. So these numbers can jump up to 10 or whatever and not trigger any diagnostic scanner "code" reader codes. Unless a very advanced reader, which can see these numbers, and also capture real time data from a short test drive.
So basically all this means, this ultra gauge will show what the Short Term Fuel Trim is & what the Long Term Fuel Trim is. Meaning when idle what is short and when driving fast long distance what is it then. PURPOSE???? If you have an air leak or some other issue causing you to burn more gas, get worse gas mileage and lose performance is likely a trim issue, cause of an air leak issue or catalytic converter sensor issue, since that thing reads what is coming out of the engine and then tells the computer what the trim should be.
see pics- ebay wireless 2000a156, 55, 54,, 50
it could be a bad fuel pump, not delievering enough fuel, pressure, could be injectors, air leaks, map, mapf or 02 oxygen sensor in manifold after engine...
Youtube video "Secret of Engine Problem Diagnosis - Fuel Trims Pt. 1" channel is Schrodingers Box
SHORT TERM FUEL TRIM (STFT)
LONG TERM FUEL TRIM (LTFT)
AND THE OXYGEN SENSOR (02)
ALL WORK TOGETHER WITH THE CARS COMPUTER. stft and ltft adjust the amount of fuel that is delivered by the injectors, and the o2 sensor is down stream of the engine in the exhaust after the manifold and measures in voltage 100-900 milivolts what the engine is outputting, if it is to rich (to much fuel) or to lean (to little fuel) then the 02 sensor metering signal will be seen by the computer and the computer will adjust the stft then the ltft (ltft is a learned metric over time, stft is a quick adjustment) long term sets a new baseline.
so briefly a rich condition will show a negative stft. from -25% to 25% should be at zero plus or minus about 5% normally. the ltft will see the problem and fix it with more or less fuel and set a new base line of fuel needed for the 02 sensor to read what it wants to see, so if there is a problem the LONG TERM FUEL TRIM will be the base line number to check if not at zero.
spark,
fuel pressure
air vacuum leak (is not measured by map or maf intake air temp iat. sensors
air leaks in, o2 sensor reads it leans things out so see drop to lean
enter fuel trims to correct this condition
Long Term Fuel Trim
Short Term Fuel Trim
at cold start the egine needs to be rich anyway, but it doesn't matter the oxygen sensors wont even measure it.
so to fix this the computer will use STFT Short Term Fuel Trim
it is shown on a scan tool over time as a percent, to maintain stoicia metric ratio.
stft should be at zero, with a leak it will increase and go positive. will be between 50 to negative 50,
anytime see stft or long term fuel trim go positive it is a response to a lean condition,
if see the fule trims go negative it is a response to a rich condition
and that is going to tell the computer to reduce injector pulse length, and reduce the amount of fuel delivery,
the o2 sesnor with a unmetered air leak will bounce normally thern go flat line then return to normal as we get more and more fuel,
once that happens the stft will level off,
and the computer will remember (memorized, learned by the computer) that for next time that that was the amount needed, to add, to return a stoicia metric ratio, (in o2 sensor readings) and that will change the long term fuel trim, as a learned metric needed, it sets a new zero point for the fuel delivery, so if need 25% more the pc will know it needs to add 25% more fuel until the condition is fixed,
o2 sensor lower voltage, it is detecting lean, (if you pull a vacuum tube)
short term fuel trim will immediately jump up, adding fuel, until the 02 sensor returns to normal some and then eventually resumes normal, check engine light wont trigger until about 25% error is detected, for lean condition,
next the long term fuel trim will start to take notice and begin to raise up some as well, because it is a learned response, so on a scale over time they both will be raised up to 20% or whatever and then the o2 sensor will also return to a wave up and down up and down not flat line,
but that means that we no longer have to call for the addition of fuel from the stft, because the ltft has set that 20% needed, as the new baseline, so the stft will drop, coming back down to zero, and the oxygen sensor continues on its marry way,
so the thing is there is still something wrong with this car,
1.) Even with the stft at zero,
2.) and the O2 sensor at 'zero', looks great
3.) We are still adding by default 25% more fuel, to compensate for the vacuum leak.
-if you now plug back in the vacuum line you disconnected... since we are at a new base line of zero on stft, when we plug that back in we are going to have a rich condition,
4.) then the stft will go negative, because it is calling for the reduction of fuel. why, because the ltft computer is adding 25% more at its new base line by dafault.
SO A RICH CONDITION WILL SHOW A NEGATIVE STFT (it will start to go negative)
because the o2 oxygen sensor is going to start going rich, raised up not flat line,
5.) So stft will go to negative 25% and then eventually the ltft will also drop,
6.) Then the o2 sensors will return to normal, we are back at stoiciametry, 14-7-1
7.) So ltft will return to factory default zero percent, and then the stft will raise up to zero again,
so to diagnose stuff like a check engine light code p0171, which is bank 1 lean (bank 1 is the fuel rail delivery system on a 4 cylinder, bank 2 is for v6 or v8 engines)
it is actually easier to make a diagnoses on a V engine because each side its own stft and ltft bank 1 bank 2.
p0171 will read something like "oxygen sensor bank 1 lean" means the 02 sensor is reporting a lean condition, and therefore the ltft must be high enough that it set this code, but it doesn't mean you need a new oxygen sensor if it is a normally operating and see a stft at zero,
1.) So ltft at 25%, stft at 0% and 02 sensor a normal wave.
..so the engine is running lean, could a dirty maf cause this yes, or a lack of fuel delivery, yes, could a weak fuel pump not delivering enough fuel causing a lean condition, yes,
2.) So how do you know you have a vacuum leak? unmetered air, all other air is coming in through the air intake, and sensors, map sensor, maf sensor, iat,
if the engine is at idle, there is fairly little air coming in through the throttle, the plate on the throttle body is basically closed tight except the slim gaps around it, and so the vacuum leak air plays a fairly significant role, in contributing unmetered air, to the total amount of air in the engine,
so if we open the throttle plates and get way more air into the engine, via increasing the engine rpm, see on scan tool increase in the throttle position sensor, now the vacuum leak plays a smaller and smaller and smaller percentage role in the total amount of air in the engine, which means less fuel trim is required, to compensate for the vacuum leak, so lets look at that on a graph.
1.) So idleing the engine and the ltft is at 25%
2.) Increase the rpm then tail off,
if it was a vacuum leak as we increase the rpm, less air of leak less significant, which means less trim is required, so the stft will go down, and that will pull down the ltft and we will see an improvement, in the ltft the more and longer that we increase rpm,
3.) Might even see that our fuel trim is at zero.
4.) Then get off the throttle and return to idel we see that the ltft returns back to 25%, so if see a pattern of rpm up ltft down, it is most likely a vacuum leak.
5.) How to diagnose? Look at the stft and use propane spray it around the eninge and the stft which should be at zero, will drop. as soon as the propane is sucked into the enigne, through the part that is leaking,
6.) Can use smoke machine, or carburator cleaner. (propane can cause fire, use it quickly and in small low setting, will hear the engine noise change if it sucked in, rpm change, can use water soap and stuff...)
-----------------document 2...
Fuel Trim Explained
I bought an Ultra Gauge that shows my fuel trim both short and long term. This post is all the information I could find on furl trims and is not currently edited for readability.
Chevy Cobalt's have one O2 Bank Sensor (for the V4 four cylinder engines), other vehicles (V6's or V8's) have 2 banks of oxygen sensors and trims associated with that. So in the picture below you see
Srt tr1 and Lng tr1 Stands for
Short Term Fuel Trim (-O2 Bank 1-) &
Long Term Fuel Trim (-O2 Bank 1-) I think
Anyway, a alarm with your OBDII sensor onboard computer will not trigger until things get to around 25% incorrect. So these numbers can jump up to 10 or whatever and not trigger any diagnostic scanner "code" reader codes. Unless a very advanced reader, which can see these numbers, and also capture real time data from a short test drive.
So basically all this means, this ultra gauge will show what the Short Term Fuel Trim is & what the Long Term Fuel Trim is. Meaning when idle what is short and when driving fast long distance what is it then. PURPOSE???? If you have an air leak or some other issue causing you to burn more gas, get worse gas mileage and lose performance is likely a trim issue, cause of an air leak issue or catalytic converter sensor issue, since that thing reads what is coming out of the engine and then tells the computer what the trim should be.
see pics- ebay wireless 2000a156, 55, 54,, 50
it could be a bad fuel pump, not delievering enough fuel, pressure, could be injectors, air leaks, map, mapf or 02 oxygen sensor in manifold after engine...
Youtube video "Secret of Engine Problem Diagnosis - Fuel Trims Pt. 1" channel is Schrodingers Box
SHORT TERM FUEL TRIM (STFT)
LONG TERM FUEL TRIM (LTFT)
AND THE OXYGEN SENSOR (02)
ALL WORK TOGETHER WITH THE CARS COMPUTER. stft and ltft adjust the amount of fuel that is delivered by the injectors, and the o2 sensor is down stream of the engine in the exhaust after the manifold and measures in voltage 100-900 milivolts what the engine is outputting, if it is to rich (to much fuel) or to lean (to little fuel) then the 02 sensor metering signal will be seen by the computer and the computer will adjust the stft then the ltft (ltft is a learned metric over time, stft is a quick adjustment) long term sets a new baseline.
so briefly a rich condition will show a negative stft. from -25% to 25% should be at zero plus or minus about 5% normally. the ltft will see the problem and fix it with more or less fuel and set a new base line of fuel needed for the 02 sensor to read what it wants to see, so if there is a problem the LONG TERM FUEL TRIM will be the base line number to check if not at zero.
spark,
fuel pressure
air vacuum leak (is not measured by map or maf intake air temp iat. sensors
air leaks in, o2 sensor reads it leans things out so see drop to lean
enter fuel trims to correct this condition
Long Term Fuel Trim
Short Term Fuel Trim
at cold start the egine needs to be rich anyway, but it doesn't matter the oxygen sensors wont even measure it.
so to fix this the computer will use STFT Short Term Fuel Trim
it is shown on a scan tool over time as a percent, to maintain stoicia metric ratio.
stft should be at zero, with a leak it will increase and go positive. will be between 50 to negative 50,
anytime see stft or long term fuel trim go positive it is a response to a lean condition,
if see the fule trims go negative it is a response to a rich condition
and that is going to tell the computer to reduce injector pulse length, and reduce the amount of fuel delivery,
the o2 sesnor with a unmetered air leak will bounce normally thern go flat line then return to normal as we get more and more fuel,
once that happens the stft will level off,
and the computer will remember (memorized, learned by the computer) that for next time that that was the amount needed, to add, to return a stoicia metric ratio, (in o2 sensor readings) and that will change the long term fuel trim, as a learned metric needed, it sets a new zero point for the fuel delivery, so if need 25% more the pc will know it needs to add 25% more fuel until the condition is fixed,
o2 sensor lower voltage, it is detecting lean, (if you pull a vacuum tube)
short term fuel trim will immediately jump up, adding fuel, until the 02 sensor returns to normal some and then eventually resumes normal, check engine light wont trigger until about 25% error is detected, for lean condition,
next the long term fuel trim will start to take notice and begin to raise up some as well, because it is a learned response, so on a scale over time they both will be raised up to 20% or whatever and then the o2 sensor will also return to a wave up and down up and down not flat line,
but that means that we no longer have to call for the addition of fuel from the stft, because the ltft has set that 20% needed, as the new baseline, so the stft will drop, coming back down to zero, and the oxygen sensor continues on its marry way,
so the thing is there is still something wrong with this car,
1.) Even with the stft at zero,
2.) and the O2 sensor at 'zero', looks great
3.) We are still adding by default 25% more fuel, to compensate for the vacuum leak.
-if you now plug back in the vacuum line you disconnected... since we are at a new base line of zero on stft, when we plug that back in we are going to have a rich condition,
4.) then the stft will go negative, because it is calling for the reduction of fuel. why, because the ltft computer is adding 25% more at its new base line by dafault.
SO A RICH CONDITION WILL SHOW A NEGATIVE STFT (it will start to go negative)
because the o2 oxygen sensor is going to start going rich, raised up not flat line,
5.) So stft will go to negative 25% and then eventually the ltft will also drop,
6.) Then the o2 sensors will return to normal, we are back at stoiciametry, 14-7-1
7.) So ltft will return to factory default zero percent, and then the stft will raise up to zero again,
so to diagnose stuff like a check engine light code p0171, which is bank 1 lean (bank 1 is the fuel rail delivery system on a 4 cylinder, bank 2 is for v6 or v8 engines)
it is actually easier to make a diagnoses on a V engine because each side its own stft and ltft bank 1 bank 2.
p0171 will read something like "oxygen sensor bank 1 lean" means the 02 sensor is reporting a lean condition, and therefore the ltft must be high enough that it set this code, but it doesn't mean you need a new oxygen sensor if it is a normally operating and see a stft at zero,
1.) So ltft at 25%, stft at 0% and 02 sensor a normal wave.
..so the engine is running lean, could a dirty maf cause this yes, or a lack of fuel delivery, yes, could a weak fuel pump not delivering enough fuel causing a lean condition, yes,
2.) So how do you know you have a vacuum leak? unmetered air, all other air is coming in through the air intake, and sensors, map sensor, maf sensor, iat,
if the engine is at idle, there is fairly little air coming in through the throttle, the plate on the throttle body is basically closed tight except the slim gaps around it, and so the vacuum leak air plays a fairly significant role, in contributing unmetered air, to the total amount of air in the engine,
so if we open the throttle plates and get way more air into the engine, via increasing the engine rpm, see on scan tool increase in the throttle position sensor, now the vacuum leak plays a smaller and smaller and smaller percentage role in the total amount of air in the engine, which means less fuel trim is required, to compensate for the vacuum leak, so lets look at that on a graph.
1.) So idleing the engine and the ltft is at 25%
2.) Increase the rpm then tail off,
if it was a vacuum leak as we increase the rpm, less air of leak less significant, which means less trim is required, so the stft will go down, and that will pull down the ltft and we will see an improvement, in the ltft the more and longer that we increase rpm,
3.) Might even see that our fuel trim is at zero.
4.) Then get off the throttle and return to idel we see that the ltft returns back to 25%, so if see a pattern of rpm up ltft down, it is most likely a vacuum leak.
5.) How to diagnose? Look at the stft and use propane spray it around the eninge and the stft which should be at zero, will drop. as soon as the propane is sucked into the enigne, through the part that is leaking,
6.) Can use smoke machine, or carburator cleaner. (propane can cause fire, use it quickly and in small low setting, will hear the engine noise change if it sucked in, rpm change, can use water soap and stuff...)
-----------------document 2...
Fuel Trim Explained
1.) Fuel Trim
Fuel Trim is the adjustment the engine computer (PCM) makes to the fuel mixture to maintain a balanced air/fuel ratio. Fuel trim is usually displayed as a PERCENTAGE reading on a scan tool.
Fuel Trim is the adjustment the engine computer (PCM) makes to the fuel mixture to maintain a balanced air/fuel ratio. Fuel trim is usually displayed as a PERCENTAGE reading on a scan tool.
For lowest emissions, the engine computer tries to keep the fuel mixture
balanced around 14.7 to 1 (14.7 parts of air to one part fuel, this is called
the stoicho metric or stoichoimetry 14:7-1). If the air/fuel ratio is less than
14.7 to one (say 12 to 1), the fuel mixture is RICH. A rich fuel mixture can
produce more power (up to a point) but it also increases fuel consumption and
emissions. Conversely, if the fuel mixture is greater than 14.7 to one (say 16
to one), it is LEAN. A lean fuel mixture reduces fuel consumption but can also
increase emissions if the air/fuel mixture is so lean that it fails to ignite
and causes lean misfire.
The engine computer monitors the air/fuel ratio via the oxygen sensor(s) in
the exhaust manifold(s). An oxygen sensor is essentially a RICH or LEAN
indicator. When the engine is running lean (too much air and not enough fuel),
the O2 sensor generates a low voltage signal that tells the engine computer
more fuel is needed. When the engine is running rich (too much fuel and not
enough air), the O2 sensor produces a higher voltage signal that tells the
engine computer the engine is getting too much fuel and to cut back the fuel
delivery. On vehicles that have an Wide Ratio Air/Fuel sensor (WRAF) or A/F
sensor, the sensor tells the computer the exact air fuel sensor so the computer
can increase or decrease the fuel delivery as needed.
Accurate fuel trim values require an accurate feedback signal from the
Oxygen sensor, otherwise the engine computer has no way of knowing whether the
fuel mixture is running rich or lean.
When a cold engine is first started, it may take 10 to 30 seconds or more
for the heaters inside the oxygen sensors to warms the sensors up to operating
temperature. Until that point is reached and the fuel feedback control system
goes into "closed loop", the fuel mixture is fixed at a predetermined
value so no fuel trim adjustments are made. But once the Oxygen sensors are hot
and the coolant temperature is high enough for the computer to go into closed loop,
the computer starts to generate fuel trim values and make adjustments in the
fuel mixture.
When the engine is shut off, the fuel trim values are retained in the
computers memory so the next time the vehicle is driven it can pick up where it
left off. Erasing the computers memory with a scan tool or by disconnecting the
battery or the PCM power supply to clear codes also wipes the fuel trim values,
which means the computer has to start learning the fuel adjustments all over
again the next time the engine runs.
2.) How to Read Fuel Trim
The fuel trim value is read by plugging a scan tool into the OBD II
diagnostic connector located under the instrument panel (on the drivers side
near the steering column). When the key is turned on, the scan tool will
initialize and start to communicate with the vehicles onboard computer.
Depending on the tool and the vehicle, it may be necessary to enter the vehicle
year, make, model and engine VIN code before the scan tool can read the data.
The engine must be started and running to read the fuel trim information.
Depending on the scan tool and how its menu options are set up, you choose the
option that allows you to read system live data. This will display a long list
of sensor outputs and other readings called PIDs (Parameter IDs). On this list
will be two fuel trim values for inline four and six cylinder engines, and four
fuel trim values for V6 and V8 engines (one pair for each cylinder bank).
There are two types of fuel trim values shown:
Short Term Fuel Trim (STFT) is what the engine computer is doing to the fuel mixture right now.
This value changes rapidly and can bounce around quite a bit depending on
engine load, speed, temperature and other operating conditions). Values normally range from negative 10
percent to positive 10 percent, though the readings may jump as much as 25
percent or more in either direction.
Long Term Fuel Trim (LTFT) is a longer term average of what the engine computer has been doing to
balance the fuel mixture over a predetermined interval of time. This value is a more accurate indicator of
how the fuel mixture is being corrected to compensate for changes in the
air/fuel ratio that are occurring inside the engine.
STFT B1 is Short Term Fuel Trim engine cylinder Bank 1
STFT B2 is Short Term Fuel Trim engine cylinder Bank 2 (V6, V8 engines, not 4 cylinder)
LTFT B1 is Long Term Fuel Trim engine cylinder Bank 1
LTFT B2 is Long Term Fuel Trim engine cylinder Bank 2 ((V6, V8 engines, not 4 cylinder)
How do you know which cylinder bank is 1 or 2 on a V6 or V8 engine? Bank 1 will be the cylinder bank that has cylinder number one in the engine firing order. For more information on firing orders, see the following:
Firing Orders (Chevy)
STFT B2 is Short Term Fuel Trim engine cylinder Bank 2 (V6, V8 engines, not 4 cylinder)
LTFT B1 is Long Term Fuel Trim engine cylinder Bank 1
LTFT B2 is Long Term Fuel Trim engine cylinder Bank 2 ((V6, V8 engines, not 4 cylinder)
How do you know which cylinder bank is 1 or 2 on a V6 or V8 engine? Bank 1 will be the cylinder bank that has cylinder number one in the engine firing order. For more information on firing orders, see the following:
Firing Orders (Chevy)
3.) What Fuel Trim Values Mean:
POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engines air/fuel mixture is running too lean.
POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engines air/fuel mixture is running too lean.
NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing
the pulse width or on-time of the fuel injectors) to reduce the amount of fuel
injected into the engine. This is done to LEAN out the fuel mixture to
compensate for what it perceives as a rich running condition.
Remember, all this is based on what the oxygen sensors are telling the
engine computer. If the O2 sensors indicate LEAN, the computer adds fuel and
generates a POSITIVE fuel trim value. If the O2 sensors are reading RICH, the
computer compensates by subtracting fuel and generates a NEGATIVE fuel trim
value.
By reading the STFT and LTFT fuel trim values on a scan tool while your
engine is running, you can tell if the air/fuel mixture is running rich
(negative fuel trim percentages) or lean (positive fuel trim percentages).
4.) What Fuel Trim Values Should Be:
Ideally, the STFT and LTFT should be within a few percentage points of zero when the engine is idling or being held at a steady RPM. Remember, STFT can bounce around quite a bit as when you suddenly snap open the throttle or decelerate. But LTFT can tell you if the average fuel/mixture is running rich or lean.
Ideally, the STFT and LTFT should be within a few percentage points of zero when the engine is idling or being held at a steady RPM. Remember, STFT can bounce around quite a bit as when you suddenly snap open the throttle or decelerate. But LTFT can tell you if the average fuel/mixture is running rich or lean.
Good LTFT values should be as close to zero as possible, though they can
range from 5 to 8 percent depending on the condition of the engine. If the LTFT
is getting up around 10 percent or higher, it usually indicates a problem that
needs to be diagnosed.
LTFT values that get up around 20 to 25 percent will usually set a P0171 or
P0174 lean code.
LTFT values that drop down to negative 20 to 25 will usually set a P0172 or
P0175 rich code.
(-obd2 scanner picture-)
This scantool is displaying a STFT value of 25 percent. Normally that would indicate a problem, but in this case the engine is not running (Engine RPM is zero). As soon as the engine starts and goes into closed loop, the fuel trim readings will begin to change.
This scantool is displaying a STFT value of 25 percent. Normally that would indicate a problem, but in this case the engine is not running (Engine RPM is zero). As soon as the engine starts and goes into closed loop, the fuel trim readings will begin to change.
5.) How Fuel, Ignition and Engine Problems Affect Fuel Trim
Lean fuel mixtures are a more common problem than rich fuel mixtures, though either can happen depending on the cause. LEAN fuel mixtures will generate higher than normal POSITIVE fuel trim readings on your scan tool. RICH fuel mixtures will generate NEGATIVE fuel trim values.
Lean fuel mixtures are a more common problem than rich fuel mixtures, though either can happen depending on the cause. LEAN fuel mixtures will generate higher than normal POSITIVE fuel trim readings on your scan tool. RICH fuel mixtures will generate NEGATIVE fuel trim values.
Some possible causes of LEAN fuel mixtures include:
Air or vacuum leaks in the intake manifold, near the throttle body or at vacuum hose connections.
Air or vacuum leaks in the intake manifold, near the throttle body or at vacuum hose connections.
Weak fuel pump that is not generating enough pressure or volume
Fuel line restrictions (like a pinches hose or plugged filter)
A weak fuel pressure regulator that is not maintaining adequate fuel
pressure
Air leaks in the PCV plumbing
Dirty MAF (Mass Airflow) sensor that is under reading airflow into the
engine
Dirty or dead fuel injectors
Ignition misfire (a fouled spark plug, weak ignition coil or bad plug wire
that causes a misfire allows unburned oxygen to pass into the exhaust and fool
the O2 sensors)
Compression leaks (bad exhaust valve that allows unburned oxygen into
exhaust and fools O2 sensors)
Exhaust manifold crack or gasket leak (allows unburned air into exhaust and
fools O2 sensors)
Bad O2 sensor (signal shorted to ground so the sensor reads lean all the
time)
Some possible causes of RICH fuel mixtures include:
Leaky fuel injector
Leaky fuel injector
Excessive fuel pressure due to bad fuel pressure regulator or restricted
fuel return line
Extremely dirty air filter or restrictions in air intake system
Exhaust restrictions (clogged converter, crushed exhaust pipe or plugged
muffler)
Bad O2 sensor (output shorted to voltage so it reads RICH all the time)
6.) Using Fuel Trim to Diagnose Problems:
Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high as plus or minus 8, but closer to zero is best. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN. Rev the engine to 1500 to 2000 RPM and hold it steady for half a minute or so. If the fuel trim numbers drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. This is because vacuum leaks have less of a leaning effect on the fuel mixture as engine speed and load increase.
Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high as plus or minus 8, but closer to zero is best. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN. Rev the engine to 1500 to 2000 RPM and hold it steady for half a minute or so. If the fuel trim numbers drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. This is because vacuum leaks have less of a leaning effect on the fuel mixture as engine speed and load increase.
If the fuel trim readings do not change much, the lean fuel condition is
more likely due to a fuel delivery problem (weak fuel pump, restricted fuel
filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum
leak.
LTFT fuel trim readings that are trending high might also be the result of
a slight ignition misfire that is not bad enough yet to set a misfire code but
is bad enough to cause a drop in fuel economy. One or more fouled spark plugs
that are misfiring occasionally, or a weak ignition coil or bad plug wire that
is allowing some occasional misfires could be the cause. For more information
on misfire diagnose, Click Here.
You can use fuel trim to identify dirty fuel injectors. If the LTFT fuel
trim readings are trending up (POSITIVE), it means the fuel feedback control
system is compensating for an air/fuel mixture that is becoming progressively
leaner over time. The most likely cause would be dirty fuel injectors. Fuel
delivery can be restricted by the accumulation of varnish deposits inside the
injector nozzles. The fix here is to clean the injectors. If the fuel trim
values return to normal after the injectors have been cleaned, it verifies you
have solved the problem. If the fuel trim values don't change after cleaning
the injectors, the lean fuel condition may be due to low fuel pressure or
air/vacuum leaks.
You can use fuel trim readings to check the response of the oxygen sensors
and engine computer to changes you make in the fuel mixture. While the engine
is idling, temporarily disconnect a vacuum hose. You should see the STFT fuel
trim readings jump immediately and go POSITIVE, and the LTFT should start to
creep up in response to the artificial lean fuel mixture you have just created
by disconnecting the vacuum hose.
To test a rich response, you can feed some propane vapor from a small
propane tank into the throttle body or a vacuum hose connection on the intake
manifold. This time, you should see a drop in fuel trim readings, with STFT
going NEGATIVE, and LTFT creeping downward in response to the rich fuel
mixture.
No change in fuel trim readings when you create an artificial lean or rich
fuel mixture would tell you the engine computer is NOT operating in closes
loop, or that the oxygen sensor(s) are not responding to changes in the fuel
mixture.
from http s://w ww.aa1car .com/library/what_is_fuel_trim.htm
LINK: Secret of Engine Problem Diagnosis- Fuel Trims Pt.1
https://www.youtube.com/watch?v=5WnM_NsOtd8
Link: Secret of Engine Problem Diagnosis- Fuel Trims Pt.2
https://www.youtube.com/watch?v=cARQ0jZZ4Qc