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Technical Information
General Fuel Trim Information and ECU Fuel Compensation Methods
The following is just general information regarding fuel trims and fuel adjustment, and minor differences may exist between different vehicle manufacturers. This information is not make or model specific, and is just the basics behind ECU fuel compensation, primarily for OBD2 vehicles. The differences between manufacturers is the reason why getting your vehicle dyno tuned by someone with experience on that particular type of vehicle is valuable, as they will be able to work more efficiently, accurately, and quickly. It is also valued if that tuner has experience with a particular engine management system as they will use their time tuning, as opposed to learning the software.
Fuel trims are the changes made to the ECU fuel maps in order to return the air fuel ratio back to its ideal position, depending on the condition. Positive (+) fuel trims (FT) indicate fuel being added to correct a lean condition, and negative (-) FT's indicate fuel being removed to correct a rich condition. FT's are expressed as a (+) or (-) percentage and should be visible on most OBD2 hand held scanners.
Short-Term Fuel Trim
The short-term fuel trim (STFT) value is dynamically calculated, depending primarily on O2 sensor feedback. The STFT value is not recorded to the ECU's memory, and is temporarily stored in a type of random access memory until the vehicle is shut off. The STFT is what allows a vehicle to adapt to different situations and atmospheric conditions without issue. The STFT correction values are only used in closed loop operation, and are not part of open loop calculations. This means that whatever correction value the STFT calls for during regular driving is not present during WOT acceleration.
Long-Term Fuel Trim
The long-term fuel trim (LTFT) value is recorded to the ECU's memory, and becomes part of the base fuel map calculations. This value is not dynamically calculated and is only altered when the STFT indicates a consistently + or - value, in which case that value will be removed from the STFT temporary memory and placed in the LTFT permanent memory. Depending on the vehicle manufacturer, the LTFT value is only altered either:
after 'X' amount of time goes by with the STFT showing a consistent (+) or (-) value
after 'X' amount of on/off cycles (vehicle actually being turned on and off) with the STFT showing a consistent (+) or (-) value
a combination of the two
After a preset (+) or (-) limit, the ECU will signal that there is a problem via the malfunction indicator light (MIL), also referred to as a check-engine light. In most vehicles this threshold is a LTFT of +/-20%. The LTFT value is used in both open and closed loop calculations. In open loop operation the LTFT value is used on its own, however the LTFT is used in conjunction with the temporary STFT value during closed loop operation.
Overall, the combination of LTFT and STFT allow a vehicles ECU to operate optimally under different circumstances, in different conditions, at varying degrees of engine and sensor degradation. After a modification is preformed many people perform an 'ECU reset' by disconnecting the battery, with hopes of allowing the ECU to 'relearn' their new modification. This actually just deletes the LTFT settings, which forces the ECU to relearn its previous LTFT values along with its ideal LTFT values for the new modification. If this 'reset' was not performed, the ECU would only have to make corrections for the new modification. LTFT values may be restored in as little as one 30 minute driving session, or take as long as 3 engine on/engine off cycles, after operating for a pre-determined amount of time.
It is recommend that after a modification is preformed; the vehicle should be driven for three 30 minute sessions in closed loop, before testing the actual modification. This will assure that the ECU had enough time to properly calculate the LTFT compensation values. Three 30 minute sessions is just an inflated estimate for the length of time it takes for an ECU to record the LTFT to its memory. If the exact time is known through data-logging or some other engine management software, then disregard the three 30 minute sessions and perform whatever known time interval is required. This can be as little as one 30 minute session in some vehicles. If this is not preformed, then the testing results are not accurate during WOT readings because the STFT correction value is ignored and previous LTFT values are used.
Chris De Freitas
www.stagefourmotorsports.com
info@stagefourmotorsports.com
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