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Reference - Misfire diagnosis checklist
Compliments of wilson009 @ bimmerfest.com
2-8-2010
See the attached PDF's for a formatted printable version
The DME can usually identify which cylinder is misfiring, It will also shut off fuel to that one cylinder for bad misfire to prevent damage and throttle the engine.
754i Misfire CheckList:
Erratic engine idle or misfiring lasting 20 seconds causes the Check Engine to be illuminated and a automatic reduction in power to prevent damage. .
First determine which Cylinder(s) are misfiring and get the error codes. The error codes will point you to the primary causes
Misfire Codes
There are three main causes for misfires as follows:
- Ignition
• spark plugs – Swap with good cylinder
o Inspect for built up, oil or smell of gas
• faulty ignition coils-– Swap with good cylinder
• faulty camshaft position sensor
• Look for loose or dirty connections
• Get a sparkplug tester and look for spark
• Use Ohm meter to test continuity
Fuel
• dirty injectors- Inspect and swap with good cylinder
• Verify it is spraying- Note computer may shut of gas to misfiring cylinder to protect the catalytic converter
• Low fuel pressure, faulty pressure regulator
• Fuel supply faulty (filter ad pump)
• Use Multimeter to test continuity and voltage
Compression
• Do a compression Test
• Loss of compression could be
o worn out piston rings (look for blue smoke)
o leaking head gasket look for (white smoke)
o Sticky, Stuck, or improperly seated valves
Vacuum
• Leaking air intake system
• Bad CCV’s
Smog
• Oxygen Sensors
• Stuck EGR valve
Computer
• DME Module replacement (Highly unlikely)
Additional Notes
Valvetronic Issues (I think this mainly deals with cold weather starting misfires)
• Incorrect tolerances and geometry of Valvetronic intermediate levers. Intermediate levers are causing uneven cylinder filling during the transitional cold start period (60 seconds after the engine has been started) when the Valvetronic system is switching from the initial 6 mm to a minimum 0.3/0.8 mm valve lift.
