Earlier today, while riding around in my friend's newly acquired 88 300ce, we noticed a distinctly weird sound coming from under the hood. At first, we thought that it might be a broken exhaust manifold or valve. We pulled over at the nearest safe area to investigate and found that the ignition wire for cylinder #3 was dangling near the exhaust manifold with what apparently looked like the electrical connector end of a spark plug! Luckily, my small toolbox was in the trunk with some spare plugs, so we were on our way a few minutes later.

My question is what can cause this, because I've never heard of a spark plug falling apart like that! I've seen the side electode breaking off before when ignition is advanced too far, but if I remember correctly, there's no way to adjust this on these engines. Besides, we had just changed the plugs in the morning and were running a fresh set. Anyway, here are some pictures...

The thing in the middle was still attached to the dangling wire


Here's a view of the spark plug. Notice that the entire center electrode is gone. In fact, I can look through the spark plug to the other end!