predator · severity: critical
AFR / Lambda Mistakes
AFR (lambda x 14.7) is the most-monitored tuning parameter and the most lethal when wrong. Petrol WOT lives in 11.5-12.5:1 (lambda 0.78-0.85). Lean spikes at high boost destroy pistons in seconds. Diesel has different ranges (lambda 1.3-1.6 cruise, 1.05-1.10 peak) but the EGT consequence is identical.
Symptoms
- • EGT exceeds 950C / 1750F sustained
- • Spark plug porcelain white (lean) or sooty black (rich)
- • Fuel-trim adaptation >+/-10%
- • Hesitation under load
- • Catalyst overheating warning
Common Causes
- • Lean cruise AFR pushed too aggressive (>15.5:1 on boost)
- • Fuel pump unable to maintain rail pressure
- • Injector duty cycle >85% sustained
- • O2 sensor heater failure (open-loop fallback)
- • Wideband uncalibrated
Fix & Recovery
- • Target 11.5-12.0 AFR at WOT on petrol turbo
- • Verify fuel pressure under load (not just at idle)
- • Upgrade LPFP/HPFP if injector duty pegs
- • Calibrate wideband in fresh air monthly
- • Datalog STFT/LTFT — adjust MAF/VE table to bring trims within +/-5%
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Prevention
- • Always install wideband BEFORE tuning, not after
- • Verify fuel-pump headroom matches power target (rule: 10% margin)
- • Use ethanol-content sensor on flex-fuel builds
- • Tune lean-best on cruise only, never WOT
When to Call a Tuner
If EGT pegged or knock follows a lean condition — pull over, let cool, datalog before next start. Melted pistons are common on lean-AFR + high-boost failures.
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