LTFT discussion ChatGPT Expalined

Let's ask ChatGPT about LTFT trims and why tuners turn them off and leave them off 

We ask -  Why do most GM tuners turn off LTFT?? 

Because LTFT can fight what a tuner is trying to do.

On most GM ECUs, Long-Term Fuel Trims are basically the ECU’s memory of past fueling corrections. That’s great for a stock car that needs to adapt to weather, aging parts, or bad gas… but during tuning (or on a heavily modified setup) they can cause more problems than they solve.

Here’s the breakdown 👇

1. LTFT hides bad MAF or VE calibration

Tuners want the commanded AFR to match reality without correction.

If LTFT is enabled:

  • The ECU quietly adds or removes fuel

  • Your wideband looks “fine”

  • But your MAF/VE tables are actually wrong

So you think the tune is good… until trims reset or conditions change. Turning off LTFT forces you to fix the actual fueling model.

2. Consistency and repeatability

Performance tuning cares about repeatable behavior.

LTFT can change:

  • After long drives

  • With altitude or weather shifts

  • After different fuel quality

That means today’s perfect pull might not match tomorrow’s. Disabling LTFT locks fueling to tables only, so what you tune is what you get.

3. Open-loop / WOT safety

At WOT, most GM ECUs ignore O₂ feedback anyway, but LTFT can still carry over into power enrichment.

That’s risky:

  • If LTFT learned lean → WOT goes lean

  • If LTFT learned rich → power is left on the table

Tuners prefer WOT fueling to be 100% table-driven and predictable.

4. Modified engines confuse trims

Big cams, headers, forced induction, E85, etc. can cause:

  • Erratic O₂ readings

  • Low vacuum

  • Reversion at idle

The ECU interprets that as a fueling error and LTFT starts “correcting” something that isn’t actually wrong. Disabling trims avoids the ECU chasing ghosts.

5. Emissions vs performance priorities

LTFT exists mainly for:

  • Emissions compliance

  • Long-term drivability for the average driver

Most performance builds prioritize:

  • Power

  • Stability

  • Control

So trims get sacrificed.

When not to disable LTFT

LTFT is useful if:

  • The car is mostly stock

  • It’s a daily driver

  • You want adaptability for temperature, altitude, or fuel quality

Some tuners compromise by:

  • Keeping LTFT on but limiting its authority (±5–8%)

  • Disabling it only in PE/WOT

  • Using it temporarily during MAF tuning, then turning it off

TL;DR

GM tuners turn off LTFT because:

  • It masks tuning errors

  • Reduces consistency

  • Can mess with WOT fueling

  • Causes issues on heavily modified engines

It’s not that LTFT is “bad” — it’s just not what you want when you’re chasing precise, repeatable performance.

If you want, tell me what platform you’re on (LS PCM, E38/E67, Gen V, etc.) and whether it’s NA or boosted — I can get way more specific. 🔧



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