HP Tuners datalog analysis — instant go / no-go

The single most useful thing in a VCM Scanner log is commanded AFR and wideband (actual) AFR plotted on the same chart. Commanded is what the tune asked for. Actual is what the engine delivered. When they track together, your fueling model is honest. When actual drifts lean of commanded under load, something between the request and the cylinder is failing: a maxed injector, a tired pump, or a saturated MAF.

The danger zones are always at high load and high RPM, where the gap matters most. TuneVault aligns these two channels across your pull and flags the exact RPM and load where they diverge, so you stop guessing and start looking at the right cell. It will always tell you to log a real wideband — the factory narrowband O2 only swings around stoich and tells you nothing useful under boost or wide-open throttle.

Knock retard (KR) in the log is the ECU pulling timing because the knock sensors heard something. Not all KR is detonation — rod noise, drivetrain rattle, and rough roads can trip the sensors. The skill is separating sensor noise (small, random, scattered) from real knock (repeatable, load-correlated, climbing with boost or RPM).

Real knock shows a pattern: it appears in the same cells, grows with cylinder pressure, and correlates with high IAT or aggressive timing. TuneVault looks for that signature rather than panicking at every degree of KR. When it sees load-correlated retard stacking up in your high-octane spark region, it tells you to back timing out of those specific cells and explains the copy-paste change — conservatively, because a tune that's pulling timing to survive is a tune that's already too aggressive.

Short-term and long-term fuel trims (STFT/LTFT) tell you how far the ECU is correcting away from your fuel model in closed loop. Consistent positive trims mean the engine is asking for more fuel than your MAF or VE table predicts; consistent negative trims mean the opposite. Big trims in specific load/RPM zones point you straight at the table cells that need work.

MAF saturation is its own failure: once airflow exceeds the top of the MAF transfer function, the sensor flatlines and the ECU stops seeing more air even as it arrives. The log shows MAF frequency or g/s pinned at a ceiling while load keeps climbing — a recipe for a sudden lean spike. TuneVault checks for that ceiling and for trims that reveal a MAF or VE table out of calibration, then points you at the specific region to correct.

Intake air temperature ties the whole log together. Rising IAT raises the engine's octane requirement, so a pull that was clean at 90 degrees IAT can knock at 140 — and that's not a tuning bug, it's physics. TuneVault correlates IAT against knock retard so you can tell heat-driven knock (fix with cooling or IAT-based timing pull) from genuinely over-aggressive spark (fix the table).

Reading a log well means looking at all of it together: AFR, KR, trims, IAT, MAF, and RPM as one picture, not isolated channels. That's what TuneVault automates — it surfaces the few moments in your pull that actually matter and explains them. It does not replace a professional tuner's judgment, it can only analyze the data you logged, and it never guarantees a result. Always confirm fueling with a wideband before trusting any tune under load.