E34/E36/E39 M50/S50/M52/S52 BMW Engine Coil Replacement

In a previous article, I detailed the procedure for replacing the sparkplugs and valove cover gasket on all BMW cars powered by derivates of the M50 engine. An item not mentioned, and probably should have been, are the coils, especially on older or higher mileage cars. With age and heat, the insulation breaks down. As variants of the M50 engine are found in the E36 3 series as well as E34 and E39 5 series, this article applies to any cars with these engines, ranging from the 1991 525i to the 1999 M3. Just like the spark plugs can be a source of intermittent hesitation or bogging, so too can the coils themselves. Do a simple search on roadfly.org in the appropriate forum and you will see countless tales of coil woe. In the early days of the E36, coil failure manifested itself so often that there was a push for recall by BMW. Coil problems can manifest themselves as bucking, surging, or general hesitation, often under load or part to full throttle in the 500-3000 rpm range. The dealership will tell you that the coils need to be replaced as a set, but this is not true. Many independent BMW parts retailers (like bimmerparts and bavauto) sell individual coils. Here's one way to test your cars coils if the above symptoms are indeed occurring: Replace all the spark plugs, first off. This is eliminating a variable and doing preventative maintenance at the same time. Next, purchase 1 new coil (You might want to check the shop you are buying parts from's policy on returning electrical or ignition components, it does you no good to buy 3 coils if it turns out you only have 1 bad one and you are stuck with 2 extra ones.) and replace the coil on the first cylinder closest to the front of the car. See if this cures it. If not, put it on cylinder #2. Rotate the new coil through all six cylinders. If no single replacement cures the issue, try a second coil and rotate through. This is the least expensive way. You could alway just replace all six at once for peace of mind. Now, old coils are prone to cracking, and a cracked coil can lead to serious electrical problems and can even damage the DME unit (ECU/chip) on your car. A new DME/chip from the dealer is $1000! You can also easily check the health of your coils by checking the resistance between two terminals on the coil. This is outlined in the Chilton's manula and the factory Bentley repair manual. There were two different sources for the coils used by BMW, Bosch and Zundspule. The Zundspule coils were made by a vendor called "May und Christ". These have been the more problematic ones, with peristent tales of misfiring and rough idle.