Misfire Detection — Bosch ME7.5

To protect expensive catalytic converters from thermal destruction and enforce strict EPA OBD-II / Euro 4 emissions compliance, an engine control unit must reliably detect whether every single cylinder combustion cycle successfully releases expansion energy or misfires. Because installing physical piezo-resistive in-cylinder pressure transducers on production road cars is economically and mechanically prohibitive, Bosch engineers developed a brilliant non-intrusive mathematical method: **Microsecond Crankshaft Angular Acceleration Analysis (Aussetzererkennung durch Laufunruheerfassung - LUE)**.

In the Bosch ME7.5 operating system, the Infineon C167CR microcontroller samples microsecond segment intervals from the 60-2 variable reluctance crankshaft position sensor (G28). By processing crankshaft acceleration through a normalized differential algorithm, ME7.5 pinpoints individual cylinder misfires in real time. However, this high sensitivity presents severe challenges when upgrading to aftermarket lightweight flywheels or aggressive multi-disc clutches.

code

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   BOSCH ME7.5 CRANKSHAFT ROUGH-RUNNING MISFIRE ARCHITECTURE                      │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ 60-2 Crankshaft Trigger Wheel & VR Sensor G28 ]                                              │
│               │                                                                                  │
│               ▼ Microsecond Edge Capture (CAPCOM Register)                                       │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Segment Duration Measurement: T_seg (Time to traverse 180° Crank Angle) │                    │
│   └────────────────────────────────────┬────────────────────────────────────┘                    │
│                                        │                                                         │
│                                        ▼                                                         │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Mathematical Rough-Running Index Calculation:                           │                    │
│   │ LUE_n = ( T_seg[n+1] - T_seg[n] ) / ( T_seg[n]^3 )                      │                    │
│   │ (Cubic Normalization Eliminates Dependence on Engine RPM)               │                    │
│   └────────────────────────────────────┬────────────────────────────────────┘                    │
│                                        │                                                         │
│                                        ▼                                                         │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Misfire Evaluation Filter: Threshold Comparison vs. KFLUE               │                    │
│   │ - Level 1: Emissions-Relevant Misfire (1-2% Rate over 1000 revs)        │                    │
│   │   → Steady MIL: P0300 Random / P0301-P0304 Cylinder Misfire              │                    │
│   │ - Level 2: Catalyst-Damaging Misfire (> 10% Rate over 200 revs)         │                    │
│   │   → FLASHING MIL + Cylinder Selective Injector Fuel Cutout!             │                    │
│   └─────────────────────────────────────────────────────────────────────────┘                    │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

50.1. Mathematical Physics of Segment Time Analysis (Tseg)

On a four-cylinder four-stroke engine (such as the 1.8T 20V), a combustion power stroke occurs every 180° KW (crank angle degrees):

Cylinder 1: 0°… 180° KW
Cylinder 3: 180°… 360° KW
Cylinder 4: 360°… 540° KW
Cylinder 2: 540°… 720° KW

During each 180° KW segment, the crankshaft accelerates during the expansion stroke as peak in-cylinder pressure (pmax≈ 40… 85 bar) pushes down on the piston face.

If a cylinder suffers an ignition or fueling misfire:

No combustion expansion occurs.
The piston must still compress the trapped charge, absorbing kinetic energy from the rotating flywheel mass.
The crankshaft experiences a sudden, sharp angular deceleration, causing the measured segment time (Tseg) for that specific cylinder to stretch significantly compared to neighboring cylinders.

Cubic RPM Normalization Formula

To prevent the rough-running threshold from requiring hundreds of complex 3D lookup tables across the entire rev range, Bosch normalizes the segment acceleration by the cube of the segment time:

LUEn =Tseg, n+1 - Tseg, nTseg, n3

Because angular velocity ω =πTseg, the derivative of angular acceleration dωdt naturally scales with Tseg3. This cubic normalization produces a non-dimensional rough-running index (LUE) that directly represents in-cylinder torque production regardless of whether the engine is idling at 800 rpm or racing at 6800 rpm.

50.2. OBD-II Misfire Classification & Catalyst Protection Failsafe

ME7.5 executes two distinct misfire monitoring algorithms in parallel within the Diagnostic Fault Path Manager (DFPM):

code

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                             TWO-TIER MISFIRE DIAGNOSTIC THRESHOLDS                               │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   1. Emissions-Relevant Misfire Monitoring (OBD-II Type B):                                      │
│      - Evaluation Window: Rolling buffer of 1,000 engine revolutions                             │
│      - Threshold: Cumulative misfire rate exceeding 1.5% to 2.5%                                 │
│      - Action: Check Engine Light (MIL) illuminated steady on second consecutive drive cycle.    │
│      - Diagnostic Codes: P0300 (Random Misfire) or P0301-P0304 (Specific Cylinder).              │
│                                                                                                  │
│   2. Catalyst-Damaging Misfire Monitoring (OBD-II Type A):                                       │
│      - Evaluation Window: Short rolling buffer of 200 engine revolutions                         │
│      - Threshold: Severe misfire rate exceeding 8.0% to 12.0%                                    │
│      - Action:                                                                                   │
│        a) CHECK ENGINE LIGHT FLASHES RAPIDLY at 1 Hz (Immediate Driver Hazard Alert).             │
│        b) Individual Cylinder Fuel Cutout (Zylinderabschaltung - B_lueabs):                      │
│           The ECU cuts the injector pulsewidth for the misfiring cylinder completely to ZERO.   │
│           Prevents unburned liquid fuel from entering the exhaust downpipe, where it would       │
│           spontaneously ignite on the catalytic converter substrate, melting the ceramic core    │
│           (T > 1050°C) and triggering vehicle fires!                                             │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

50.3. The Lightweight Flywheel (SMFW) False Misfire Problem

One of the most notorious drivability headaches encountered on tuned 1.8T vehicles is the Lightweight Single-Mass Flywheel (SMFW) Misfire Dilemma:

Factory Dual-Mass Flywheel (DMF): Weighs approximately 13.2 kg (29 lbs) and incorporates internal circumferential arc springs designed to isolate and dampen torsional crankshaft vibrations between the engine and transmission.
Aftermarket Single-Mass Flywheel (SMFW): Billet steel or forged aluminum units weigh between 4.5 kg and 7.5 kg (10… 16 lbs).
The Problem: Without the dual-mass rotational damping, high-order torsional vibrations from the crankshaft transmit directly into the ring gear at specific engine speeds (typically 2600… 3400 rpm under light deceleration or steady-state cruising).
The Result: The VR sensor G28 reads microsecond tooth fluctuations caused by drivetrain chatter. The ME7.5 LUE algorithm misinterprets this mechanical oscillation as consecutive cylinder misfires, illuminating a flashing Check Engine Light (P0300, P0301, P0303) and cutting fuel to individual cylinders while driving on the highway!

50.4. Core Calibration Maps & Recalibration Strategy in `06A906032LP`

To resolve false misfire cutouts when running lightweight flywheels or aggressive track clutches, recalibrate the following maps in flash:

Symbol Name · Flash Offset (032LP) · Dimensions · Axes
Symbol Name	Flash Offset (032LP)	Dimensions	Axes	Units	Calibration Function
`KFLUE`	`0x015822`	12 × 8	RPM × Load	Roughness Index	Kennfeld für Aussetzererkennungs-Schwellenwert (Primary Misfire Detection Threshold)
`KFLUEAB`	`0x015904`	12 × 8	RPM × Load	Roughness Index	Schwellenwert für Zylinderabschaltung (Threshold to trigger cylinder injector fuel cutout)
`MDLUE`	`0x015810`	1 × 1	-	Revs	Minimum continuous revolutions required to confirm misfire event
`NMLUE`	`0x015814`	1 × 1	-	RPM	Minimum engine speed for misfire detection activation

Tuning Protocol for Lightweight Flywheels

Raise KFLUE in Resonance Band: Log the raw rough-running index using high-speed ME7Logger (LUE1, LUE2, LUE3, LUE4). In the resonance window (2500… 3500 rpm at low load rl = 15… 40%), increase the roughness threshold values in KFLUE by 35… 50% above stock baseline.
Deactivate Fuel Cutout (KFLUEAB): Increase the fuel shut-off threshold table KFLUEAB to maximum safe levels (0xFFFF or +100%) in the cruising load cells. This permanently prevents the ECU from shutting off fuel injectors due to drivetrain chatter, while leaving emissions DTC logging active for genuine ignition coil failures.