Ignition - Engine Misfire/Rough Idle

85BMW03

GROUP 12 EngineElectrical
Bulletin Number 12 07 84 (906) Page 1 of 1
Montvale, NJ January 1985 Technical Dept.
SUBJECT: Engine Misfire

MODELS: All

Condition: When working on the ignition system (including replacement of spark plugs) it is essential to check whether the high voltage components, wiring, connectors and spark plug suppressors/connectors conform to the latest standards as stated in our S.I. 12 04 84 (863).

General
Information: To comply with legislation on exhaust emission control, BMW employs a very lean fuel/air mixture (lean concept).

This basically requires high ignition voltage, which is increased even further because of
a) electrode erosion taking place during operation, or
b) carbon deposits on the spark plugs.

If the ignition voltage exceeds a value greater than that which the high voltage circuit insulator can withstand, there will be a spark discharge at this point and consequently noticeable misfiring.

Replacing the spark plugs (and therefore reducing the ignition voltage requirement) will, in fact, eliminate misfiring, but in many cases, not the actual cause of the misfire.