Fuel Trim Diagnostics
A cars computer (PCM) can make adjustments to the amount
of fuel delivered to it's engine by varying the amount of fuel trim given to an
injector pulse. Fuel trim is a percentage of positive or negative correction of
fuel delivered, and it is used to keep the optimum fuel ratio to air. Short Term
Fuel Trim (STFT) is based off of the Oxygen Sensors reporting of the current
Airfuel ratio. Long Term Fuel Trim (LTFT) is used by the PCM to keep the STFT as
close to zero as it can. These Trims are reported on a per engine bank basis.
Diagnosing a vehicles fuel system can be
made easier with the use of Escans Fuel Trim Chart. When combined with the Volumetic
Efficiency Chart, patterns can be identified that may indicate problems in parts
of the fuel or Engine control systems.
Below is a chart from a vehicle with normal
fuel trim control.
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Escan did a nice job of gathering the data and putting it into this Easy to read
chart form. The LTFT is close to 0 throughout the chart, with only a difference of
about 5%. Notice the Fuel Trims are consistant from Bank 1 to Bank 2. Also note
that they are also pretty even throughout the RPM and Throttle Position or "TPS"
Range.
In this next chart below we have another vehicle which has a LTFT problem. |
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In this vehicle we notice these trims didn't keep near 0%. They have a wide spread
between the highest a the lowest of about 20%. They are consistant though from Bank1
to Bank2. It is also noteworthy that they start out Negative at Idle and turn Positive
at Higher RPM and TPS. A pattern of this sort is indicative of a dirty Mass Airflow
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Next we have a trim chart from a 1999 Ford Expedition with a 5.4 |
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This vehicle has High LTFT at lower RPM and they come down at off idle, higher
RPM. They are also even from Bank 1 to Bank 2. This type of trim pattern leads itself
to a possible vacuum leak. At idle a vacuum leak has more of an effect on an engine
than at higher RPM. After locating a leaky PCV hose we went for another test drive
and we came up with the chart below. |
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You can see here that the idle LTFT came down about 6 to 7%. The higher RPM's were
not effected. This vehicle still exhibits a higher than desired LTFT and could be
a cause of concern but the owner declined further repair/diagnosis.
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In the pic below we have a Trim Chart from a Chevy Astro van
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In this one the lower idle area LTFT is adding a significant amount of fuel. It
almost looks like this vehicle has a vacuum leak. When we ran the VE test this vehicle
read lower than normal by about 10%. Upon inspection of the intake system it was
noticed that a hose was disconnected off of the intake boot. Technically this was
considered unmetered air and not a vacuum leak. A vacuum leak would be unregulated
air brought into the engine but not through the throttle body. This unmetered air
entered the intake through the throttle body but was not measured by the Mass Airflow
Sensor. Since we had extra air in the engine, we did not have enough fuel. The feedback
system added more fuel and that is why we have higher LTFT numbers. I believe we
don't have higher numbers at the higher RPM's because the leak wasn't flowing as
much air as the throttle body was at the higher RPM's.
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These were the trims after reattaching the hose.
In this pic below a Pontiac Grand Am is experiencing high amounts of negative LTFT.
It was due to an overactive Evaporative Emmisions system sending large amounts of
fuel vapor to the engine. This amount of fuel was not as significant at the higher
RPM's hence the normal trims at the higher RPM's. |
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You can download the Escan Program here.
Escan Download
You can download a Fuel Trim Data Capture Video and Data here.
Ford Escort Fuel Trim Chart Video and Supporting Data
You can watch the filling of the Fuel Trim Chart in a short screen capture video.
Also you can then Open the Escan Program in Demo Mode and Load the Data capture (F10)
to display the data and try out the graphing.
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