If you are inclined to change your own spark plugs on your BMW there are some things you need to know before heading out to the local auto parts store . . . just cause it’ll fit, doesn’t mean that it’s correct.  There are many areas of the auto parts industry where quality and sound advice has taken a back seat to marketing and profit margins.  One of these areas is the good old spark plug.  I’m going to attempt to keep this simple and not too technical, which basically means you’ll have to trust me. (trust me, now that’s  mechanic line if I’ve ever heard one.)

When you go to the auto parts store and order spark plugs for your 1985 535i they will have a neat little diplay that has about 5 different spark plugs ranging from what looks like a normal plug to one that looks like a space probe on the end.  The guy behind the counter will tell you your options, all of them with code numbers that make no sense and he’ll eventually tell you how great the platinum ones are with the 4 prongs at the top.   STOP!  You need to go in there knowing exactly what you need, and getting only that item.  Most people will get the platinum spark plugs by the way.

BMW Service San Diego

I used the ’85 535i as it takes a special plug, the Bosch WR9LS; a bit expensive as it has a silver core and the platinum plug that fits will probably be less expensive.  This car hates any other spark plug.  Spark plugs are funny things, they will all work, but not really work all the same.  When BMW produces a car, they go through tons of engineering testing and design, work out a fuel injection and ignition system and then specify a spark plug that will work best with this combination.  The guys at the auto parts stores look at a spark plug, figure out which cars it will fit, and then offer it for sale and push the ones with the highest profit margin.

Remember that your ignition system (this controls spark) has a specific design and was also designed at a particular time in automotive history.  If you own a 1972 BMW 2002, it takes a simple spark plug, there was no such thing as a 4 prong platinum plug in the early 70’s in mass produced automobiles so don’t use it.  The plug will not be optimized and neither will your car.  Plug gaps are also of importance.  In most repair books the specified gap will be the original gap provided by the manufacturer, some of which have changed.  It had been found that if you increased the plug gap for a 1990 325i it would result in a smoother idle as well as a boost in performance.  Regardless of what year your BMW was built, spark plugs should be changed during every Inspection II Service, which is BMW talk for the major tune-up.

Here’s a great link from NGK on ‘reading’ spark plugs.  The older your BMW the more this will apply, and BMW’s built post 1995 will barely apply to this information.  Good nonetheless.

Reading your spark plugs

Chris Keefer

Independent Motorcars

5836 Autoport Mall

San Diego CA 92121