How long does the LT Fuel Trim take to mean something?

When the engineers design the ICE, they measure all the sensor inputs (everything except the Air-Fuel or O2 sensors) while figuring out how much fuel injected plus what ignition timing it takes to achieve their power and emissions goals.

That data is compiled into fuel (and ignition) "maps" that are programmed into the ecu. These can look like grid charts, RPM on one axis and load on the other. So there are values - millisec of injector on time (and x° ignition timing) for the entire expected range of RPM x load. This is the base fuel injector "on time" chart.

1000 RPM in park is different from 25 mph cruising and from "foot to the floor" on the highway. The values from the ecu maps are adjusted for other sensor inputs such as temperatures, APP (driver's "intent" - desired RPM) etc before the injectors and coils are triggered.

The Lambda (A-F and O2) sensors are part of the feedback loop that's designed to keep the ICE operating at the intended Air-Fuel ratio.

When the ecu ignores the Lambda sensor inputs, that's called "open loop" operation (such as cold start up, or decel fuel cut). When the Lambda sensors are used, that's "closed loop".

If the AF reports that the exhaust gas mixture is too lean, the ecu makes a positive correction to increase fuel (a "plus" percent of the current injector base on time). When AF reports too rich, it's a negative correction to reduce fuel (a "minus" percent).

Short Term fuel trim is a temporary correction that's a direct response to the AF sensor. ST's job is to keep the AF ratio centered on the programmed value (usually 14.7:1).

If ST is "too far away" from 0 (maybe 5%?) for "too much time" (maybe a few minutes?), then Long Term fuel trim steps in to bring ST back towards 0. LT's job is to keep ST centered near 0

LT trim is a learned correction that's stored in the ecu on its own RPM x load map. ST and LT are additive. If something causes ST to stay near +10%, LT would shift towards +10%, and ST would shift back towards 0%.

Typical range for ST and LT is around ± 25%. 0 ± 5% is ideal. ± 10% is acceptable. ± 15% means something is off but it can compensate. Over ± 20% should be setting codes or have driveability problems.

So LT trim isn't really indicating any significant historical data, just compensation for a given RPM x load - whenever the last time that the engine operated under those conditions. Might have been yesterday, or might have been last month. If ST changes too much, then LT will change. If ST stays near 0, then LT won't change.

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One drive cycle usually

SM-G960U ?

 

There are many dymanic parameters effecting this value, such as temperature, air pressure, driving habits, AfR data and more. This is a lovely explanation: