Engine Misfire Diagnosis

Engine Misfire is the result of inefficient combustion in one or more cylinders. The causes of Engine Misfire are extensive but can be grouped into the following sub-systems. Consider the charts below as an additional diagnostic aid once the DISplus/MoDIC is connected, the correct fault symptom has been chosen and the fault memory has been interrogated. Follow the Test Module as displayed by the DISplus/MoDIC.

• COMPONENT
• POSSIBLE CONDITION
• TEST
• CORRECTION
• Spark Plug: 
  • Incorrect spark plug installed
  • Electrode gap closed or too small
  • Electrode(s) missing
  • Oil or fuel fouled spark plug
  • Ceramic insulation cracked
• Secondary Ignition DISplus Preset Measurement
  • Verify correct spark plug
  • Replace if necessary
  • Swap with another cylinder
• Secondary circuit: (wiring, M73-cap, rotor) 
  • Wet or moist due to water infiltration.
  • High resistance due to corrosion.
  • Check water ingress, repair, replace
  • Check resistance value, replace
• Ignition Coil(s): 
  • Secondary/Primary Circuits open or shorted.
  • Housing cracked, damaged.
• Secondary and Primary
  • Inspect and replace if necessary
  • Swap with another cylinder
• Ignition Coil & Engine Harness Connectors 
  • Power supply, Primary control and ground (shunt signal) circuits impaired.
• Primary Ignition & Term 4A feedback Preset Measurements
  • Look for open, loose connector, corrosion, crossed or backed out pins (also consider ignition unloader or ECM relay on MY97 and newer cars).
  • Determine defective condition, repair or replace.
  • A secondary ignition oscilloscope display provides vital information about the ignition system's condition.
  • Follow the precautions in group 12 of the Repair Instructions.
  • Use the following scope patterns as a guideline for ignition system diagnosis. Use the preset measurement function of DISplus.

    Evaluation of secondary signal amplitude at idle speed.

    Fig 1: Identifying Evaluation Of Secondary Signal Amplitude At Idle Speed

    Courtesy of BMW OF NORTH AMERICA, INC.
  • COMPONENT
  • POSSIBLE CONDITION
  • TEST
  • CORRECTION
• Pistons, Rings, Valves, Camshaft: 
  • Hole in piston crown, ring(s) broken, valve(s) not seating, valve(s) bent, valve spring(s) broken, camshaft lobe cracked, etc.
  • Idle Quality - Rough Running Preset.
  • Cylinder compression & leakdown tests.
  • Correct condition as required.
• Hydraulic Valve Actuator (HVA): 
  • HVA oil bore restricted or blocked.
  • Engine oil pressure builds up too slow.
  • Intermittent Misfire Fault  - Not Currently Present.
  • HVA binding/sticking in bore.
  • Idle Quality - Rough Running Preset.
  • Listen to HVA
  • Check Oil Pressure
  • Cylinder leakdown
  • Always consider mechanical components when diagnosing misfire.
  • Inspect for scoring.
• Vacuum Leaks: 
  • Unmetered vacuum leaks causing a "lean  " operating condition. Possible "Excessive Mixture Deviation  " fault codes.
  • Idle Quality - Rough Running Preset. Test for vacuum leaks.
  • Interpret Add. & Multipl. adaptation values
  • Correct condition as required
  • COMPONENT
  • POSSIBLE CONDITION
  • TEST
  • CORRECTION
• Fuel (quality): 
  • Contaminated fuel, (water, other non combustible).
  • Clean fuel system, replace fuel.
• Fuel Delivery: 
  • Fuel pump delivery pressure low, restriction in fuel line to fuel rail or running loss valve.
  • Fuel filter restricted (clogged).
  • Low fuel in tank.
  • Check fuel pressure & volume.
  • Check fuel pump power and ground
  • Determine restriction/flow reduction, replace component as necessary.
  • Interpret Additive and Multiplicative adaptation values.
• Running Loss Valve: 
  • Valve stuck in "small circuit" position.
  • Check valve
  • Display "diagnosis requests" in DISplus and test valve for proper function, replace valve as necessary.
• Fuel Injectors: 
  • Leaking fuel injector pintle seats cause rich engine starts with hot ambient temperatures.
  • Blocked (dirty) injector(s).
  • Ti Preset & status page.
  • Sec Ign scope pattern.
  • Check injectors for leakage.
  • Swap suspect injector with another cylinder.
  • Inspect injector, replace if necessary.
• Fuel Pressure Regulator: 
  • Regulator defective, causes fluctuation in the injected quantity of fuel causing mixture adaptation faults.
  • Fuel pressure
  • Check nominal fuel pressure value with engine operating under varied speeds.
• Evaporative System: 
  • Defective evaporative system vent causing fuel tank collapse and fuel. starvation.
  • DISplus status, EVAP test with pressure tool, purge valve func. test.
  • Check the fuel tank condition and vent line.
  • Check Fresh Air Valve on TLEV E36 vehicles or LDP/DM TL and filter on ORVR vehicles for proper system "breathing".
  • COMPONENT
  • POSSIBLE CONDITION
  • TEST
  • CORRECTION
• Crankshaft Position Sensor or Increment Wheel: 
  • Implausible signal for misfire detection.
  • Increment wheel loose or damaged (internal on M44, M52 and M54, external on M62 & M73).
  • Air gap between sensor and wheel.
  • Noticeable at higher RPM.
  • DISplus preset measurement.
  • Determine defective sensor or increment wheel and replace.
• Catalyst Damaged: 
  • Excessive exhaust back pressure (bank specific fault present, more noticeable under heavy load and high RPM).
  • DISplus preset measurement of oxygen sensor.
  • Back pressure test per SIB with Special Tool.
  • Determine catalyst condition, replace or repair as necessary.
• Oxygen Sensor: 
  • Excessive mixture deviation, possible vacuum leaks. 
  • Monitor oxygen sensor signal via DIS preset.
  • Swap sensor from other bank (if applicable) and see if fault transfers to other bank.
• Engine Control Module 
  • Internal control module fault.
  • Misfire Reprogramming.
  • Check fault memory.
  • Highly unlikely but must be considered.
  • Check Model/Prod range - reprogram

  When diagnosing a Misfire fault code, Remember: 

  "Misfire" is caused by a defect in the internal combustion engine or a defect in the control of the engine operation. 

  "Misfire" is the result of  improper combustion (variation between cylinders) as measured at the crankshaft due to: 

  • Engine mechanical defects; breakage, wear, leakage or improper tolerances.
  • Excessive mixture deviation; air (vacuum leaks), fuel and all the components that deliver air/fuel into the combustion chambers.
  • Faulty ignition; primary, secondary including spark plugs.
  • Faulty exhaust flow; affecting back pressure.
  • Tolerance parameters; ECM programming.

  A Misfire fault code(s) is the "symptom" of a faulty input for proper combustion. When diagnosing a misfire, review the charts to assist you in finding the faulty input.