BMW M3 e46 Coil failure Symptoms:
Engine will have sporadic or permanent cylinder miss fire on one or multiple cylinders. This will be accompanied by a C.E.L. check engine light. At idle engine will feel rough, idle poorly and have a lack of power.
Your BMW may also experience abrupt jerking during acceleration. In many cases the miss fire will occur only after the engines warms up. Early stages will be periodic miss fire sometimes after extended driving sessions.
Locating the coils on a BMW V8 engine:
The coils on all BMW V8 engines after 1992 are located down the center line of the cylinder head under the protective plastic cylinder head trim cover. To get access you must first remove the small plastic trim caps which hide the 6mm trim plate fasteners.
Removing these 6 mm hex nut will release the plastic cover. This will release the plastic trim piece which covers the actual engine valve cover and ignition coils.
How the BMW direct fire ignition system works:
All BMW 6 and 8 cylinder engines after 1992 use a single coil which is mounted directly over each spark plug. This design eliminates the needs for ignition wires, distributor caps, rotors and the mechanisms to drive these components. Timing and spark sequence is all computer controlled. Reliability is increased due to elimination of these components. Having one coil for each cylinder eliminates a no run situation due to coil failure, since all 6 or 8 coils would need to fail at the same time. This also requires each coil to fire only once per two revolutions, rather than a single coil firing all eight cylinders.
On the older BMW models using the single coil ignition system a failed coil typically left you stopped on the road. On a BMW direct fire multi-coil system a failed coil will still allow you to drive the vehicle on the remaining functioning cylinders.
Danger of getting shocked:
The only high voltage at the coil is at the end of the connector at the spark plug. The signal connector coming from the engines wire harness is low voltage. This signal comes from the control unit. Note, coils operate at very high voltage. However the amperage is very low. Although shock from a coil is rarely fatal. It does hurt.
My recommendation is to use insulated gloves, or pliers with insulated handle.
How to check which coil is bad:
Performing this field test can only be accomplished when the coil is not functioning, or miss firing. With plastic trim cover removed from the cylinder head. Before stating the engine locate all the coils and release the hold down clips for each coils signal wire plug. Make sure you leave the signal wire plug connected to the coil. Now with the Engine idling, Start with coil number one, in the case of the V8 models select a forward coil on one bank of the V8. Visually and also by place your hand gently on the intake manifold feel and remember the shack or uneven running.
The video you see is showing a glass of water as a visual indicator. This (glass and water visualization) may not be possible on all models. With a gloved hand or insulated pliers, engine idling gently unplug the signal wire plug. If the engine begins running worst then that coil is functioning correctly. Continue testing all coils in this fashion until you find the coil that make no difference when it is unplugged. If you disconnect the coil and there is no change, then that coil is not functioning.
Confirming you findings:
Now that you have located the suspected defective coil turn off the engine, remove the bad coil and switch it with a known functioning coil from another location.
Restart the engine see if the miss fire has moved to the new location. If you are replacing only one coil you may do so at this time. However if the vehicle has extremely high mileage you may consider doing them as a set.
Replacing the coils
when replacing these coils pay close attention to the tip of the coil connector at the spark plug end. If this area is saturated in engine oil then you have a leaking valve cover gasket. Although this may or may not be casing your engine misfire the situation must be corrected. Extended oil saturation will damage the coil connectors on any of the new replacement Bosch ignition coils.