Did you ever get water under your BMW Distributor Cap? You either wait for it to dry out, or you spray some water-repelling oil in the cap to dry it out--until then, you're not going anywhere. That's because all the high-voltage electricity produced by your ignition system has to run through your distributor cap before it goes to the plugs. The ignition rotor spins beneath the cap, contacting the individual towers that the plug wires are connected to, sending the power to the right place. And it all happens hundreds of times a second.

Not too shabby for a bunch of plastic and brass, eh? The BMW distributor cap has worked well for a long time, but it's slowly giving way to distributorless ignition systems, or DIS. That's because things like water under the cap, and carbon tracking from wear, and even cracks in the plastic cap itself can cause a failure of your car's ignition system. Modern DIS systems aren't susceptible to these sorts of mechanical failures. Of course, on the other hand, you can replace a bad BMW distributor cap yourself for just a few bucks. Try that with a DIS coil pack!