Rule Zero: Diagnose Before You Modify
Start with DTCs, freeze-frame/context, a known-good battery/charging system and a saved baseline log. HP Tuners’ getting-started guidance recommends checking DTCs and establishing a baseline before calibration changes. A tune cannot repair a failing injector, vacuum leak, weak pump, mechanical problem or slipping clutch pack.
Misfire / Rough Running
- Read DTCs and identify whether the misfire is cylinder-specific or random.
- Check plugs, wire/coil connection and obvious harness damage.
- Swap a suspect coil/injector with another cylinder and see whether the symptom follows.
- Check fuel pressure and commanded/actual lambda.
- If the miss stays on the cylinder, perform compression/leak-down and inspect valvetrain motion.
Lean At WOT / Fuel Pressure Falls
- Stop the pull if lambda trends lean beyond the safe target for the combination.
- Log fuel pressure with a trustworthy sensor/gauge and verify pump voltage under load.
- Inspect filter, pickup, tank venting, wiring/relay, regulator reference and line restriction.
- Verify injector duty cycle and differential pressure—not just nominal rail pressure.
- Only change fueling calibration after supply hardware is proven stable.
Knock Retard
- Determine whether KR repeats at the same rpm/load or appears randomly with bumps/shifts.
- Verify fuel quality and commanded/actual lambda.
- Inspect for exhaust contact, loose brackets, converter/flexplate noise or other false-knock sources.
- Review spark, IAT/ECT and cylinder airmass/load context.
- If repeatable under clean conditions, reduce risk and investigate octane, mixture, chamber deposits, compression and timing—not just the knock sensor tables.
6L80 Flare / Slip / Harsh Shift
- Check transmission DTCs and fluid level/condition using the correct service procedure.
- Log commanded gear, actual gear, input/output speed, shift time, TCC slip and relevant pressure/torque parameters available for the controller.
- Separate a calibration scheduling issue from an actual ratio error or clutch slip.
- Do not “fix” mechanical slip by simply commanding more pressure or removing torque management.
Safety: WOT and transmission diagnostics must be performed in a controlled, legal environment. If oil pressure, fuel pressure, temperature or transmission behavior is unsafe, stop the test.