Engine - Misfire/Rough Idle Under Heavy load

9217bmw01

Group 12
Engine - Electric


Bulletin Number
12 02 92 (3506)


Woodcliff Lake, NJ
February 1992
Product Engineering


SUBJECT: Ignition Coil Failure

ENGINE: M50


Situation:
A complaint of rough idle and/or a misfire under heavy engine load may be received on E36 and E34 (M50 equipped) vehicles due to a break on the secondary side of the ignition coils. These coils were produced by the vendor named "May und Christ".

Correction:
If a vehicle is encountered with the above complaint proceed as follows:

- With the cylinder head cover removed (exposing the tops of the ignition coils) identify the vendor:

Vendor Identification

"May und Christ" = ZUNDSPULE
Bosch = Bosch

- Identify the production year and month of the "May und Christ" coils located directly above the word ZUNDSPULE, i.e., 91 M 06 (year = 91, month = 06 (June)). Coils with a production date code prior to 91 M 01 (January 1991) may have a break on the secondary side which can be diagnosed by removing the ignition coil from the engine.

- With the coil removed, twist and pull the spark plug connector off the coil and examine the connection.

A poor connection may be seen as green corrosion or (a) piece(s) of plastic found in the ignition coil socket which has broken off the spark plug connector. In this case the coil must be replaced.


Note: A further check can be performed when diagnosing the primary and secondary signals of an ignition coil as outlined in E36 Repair Manual microfiche, Group 12, Page 12-13/3, using the BMW Service Tester and special tool adaptor cables. However, the secondary signal scope pattern may be misleading and may not diagnose a coil which is defective under engine load, i.e., a large spike displayed on the scope screen.

Should a vehicle be encountered with a complaint of an engine miss which cannot be diagnosed using the above methods then, using a known good coil, swap coils one at a time until the defective coil is located by the process of elimination.

BMW AG is aware of this time consuming diagnostic procedure and is currently developing a new adaptor tool which will allow for a more efficient and effective diagnosis process.