ARC-TUNING — DFCO + Lean Cruise

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Scarecrow Briefing — DFCO + Lean Cruise

DFCO (Decel Fuel Cutoff) is the PCM briefly turning injectors off during coast. The engine is being driven by the wheels at that moment — combustion isn't needed. Saves fuel, reduces cat heat, makes trailing throttle clean. Lean Cruise is a separate feature: at steady-state highway cruise, run leaner than stoich (~15.5–16.5 AFR) to save fuel.

(1) DFCO is a 5-state machine — Inactive → Pending → Active → Re-engaging → Inactive. Each transition has thresholds. The KB documents all five. (2) DFCO must be DISABLED during VE tuning (spec §5.2). DFCO drops read as massive STFT events and contaminate the trim data. Re-enable when VE is done. (3) Lean Cruise is gated on cam aggressiveness (spec §5.9). If mods.cam.specs.duration_int_at_050 > 230°, lean cruise is greyed out — cam overlap dilutes cylinder charge, lean limit moves close to stoich. (4) Emissions scope matters. Lean cruise can affect NOx output. If emissions.scope === 'street-legal', the page warns; if 'race-only-private-property', no warning.

KB: dfco-decel-fuel-cutoff.md, corvetteforum-dfco-deep-dive.md, hptuners-forum-dfco-timer-delay.md, afr-target-ranges-by-load.md. Spec §5.2 / §5.9.

2.1 DFCO Enable Conditions 📍 LOCATE

HPTEngine → Fuel → Open/Closed Loop → DFCO Enable

— RPM threshold, MAP threshold, TPS threshold

DFCO triggers when: RPM > threshold (typically ~1100), TPS < threshold (typically <1%, "off the throttle"), MAP < threshold (typically <30 kPa, indicating decel/coast), and ECT high enough that the cat won't melt on re-engage. All four must be true. If any one fails, no DFCO.

When the PCM cuts fuel during coast. Conditions that all must be true: foot off the gas, engine spinning fast enough, intake vacuum high enough (you're decelerating), engine warm.

2.2 DFCO Re-Engage Threshold (timer + RPM) 📍 LOCATE

HPTEngine → Fuel → Open/Closed Loop → DFCO Disable

— When fuel comes back in

Re-engage happens at a LOWER RPM than the trigger threshold (hysteresis — prevents rapid on/off). Common: trigger at 1100 RPM, re-engage at 900 RPM. Plus a timer delay (~0.5s) to prevent re-engage during the brief lift between gear shifts. If re-engage is too aggressive, you get a fuel-on jolt during coast. Too lazy and the engine stalls when you actually need fuel back.

Fuel kicks back in at a lower RPM than where it cut out. There's also a small delay so it doesn't switch on/off during quick gear changes.

2.3 DFCO during VE tuning — DISABLE ⚠ Halt rule per spec §5.2

DFCO must be off while you're tuning VE. DFCO drops read as massive STFT swings and contaminate the trim data the VE-tune learns from. The helper inline-warns if a VE log shows DFCO active. Production behavior: a one-click "tune-time DFCO disable" preset that the user re-enables at session end.

Turn DFCO off while you're tuning the VE table. The fuel cuts mess up the trim numbers the tune is reading from.

⛔ SCARECROW HALTS HERE IF…

DFCO ACTIVE IN VE LOG If a VE log shows DFCO state cycling, the trim data is corrupted. Re-log with DFCO disabled. Spec §5.2.

2.4 Lean Cruise — Enable / Cam Gate 📍 LOCATE

HPTEngine → Fuel → Lean Cruise → Enable

⚠ Cam-aggressive gate per spec §5.9

Lean Cruise targets ~15.5–16.5 AFR at steady-state highway cruise (1500–2500 RPM, light load). Saves 5–8% fuel on the highway. Greyed out if mods.cam.specs.duration_int_at_050 > 230°: cam overlap dilutes cylinder charge, lean limit moves close to stoich, and running 16:1 with that much overlap means misfire and combustion instability. KB explicit on this.

Run leaner-than-normal AFR while cruising on the highway to save gas. Doesn't work with a big cam — the engine misfires.

⛔ SCARECROW HALTS HERE IF…

CAM > 230° @ 0.050" Lean cruise unreliable on big cams. Refused per spec §5.9. KB: afr-target-ranges-by-load.md.

EMISSIONS = STREET-LEGAL Lean cruise raises NOx slightly. Page warns but doesn't refuse. If the user owns a smog jurisdiction, advisory is theirs to weigh.

2.5 Lean Cruise — AFR Target + Window 📍 LOCATE

HPTEngine → Fuel → Lean Cruise → AFR Target + Active Cells

— RPM × MAP cells where lean cruise applies

Active cell window is typically 1500–3000 RPM × 30–55 kPa MAP (light highway load). Outside that window, normal AFR target applies. Target AFR in the window: 15.5–16.5 (gasoline, stoich = 14.7) — the leaner end is fine on a stock cam, the richer end is safer on a mild cam. Verify with wideband — narrowband O2 reads "lean" with no nuance.

Set the AFR target leaner than normal in a specific RPM/load box (highway cruise). Use a wideband to check it.

2.6 Verification Loop — Highway coast-down + steady-state cruise

Per spec §4.2: highway coast-down sections (verify DFCO state cycles correctly) + steady cruise at multiple speeds (45/55/65/75 mph) to verify lean cruise stays in target. Channels: RPM, MAP, TPS, AFR Cmd, AFR Actual, ECT, DFCO state, vehicle speed, gear (auto), brake pedal state, instant fuel rate.

Drive on the highway. Coast down hills (DFCO check). Hold steady speed at several speeds (lean cruise check). Log it.