Bosch H9DC0 Exploded!

[alienman]

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!

[420 SE]

Wow

Don't quite know what to say to that - not heard of it before but I would be taking it back if it was new and writing a letter to Bosch.

Lucky nothing dropped into the engine - that may have been untidy!!!

[Ferdman]

alienman, I've never seen a spark plug do that. Recommend removing the repacment plug and inspecting the #3 piston head and combustion chamber with a borescope to check for remnants of the first plug and damage to the piston head and/or valves. What did the other plugs look like after #3 self-destructed?

[alienman]

Hi Ferdman,

The other plugs looked ok after #3 blew. We didn't get a chance to look for remnants, as we don't have a borescope. My friend is going to get the car towed to a nearby indy via AAA tomorrow and have them check it out. I wonder if he could get a free valve job out of it

[Benzmeister]

on Saturday. One thing I did notice, you have to push those metal "cover" onto the plug firmly until you hear a pop. When I was taking them off, one by one and changin' the plugs one by one. I noticed some of them were loose. the ones that were seated correctly, I had to tug on to get off. Don't know if this has anything at all to do with what happened to your plug. The old plug were actually in pretty good shaped, carboned up, as the car had only be driven 5K in the last 15 years, I probably cound have gotten away with cleaning them, but decided on new ones. After the job was done, I noticed the idle is no very smooth, before, there was a skip now and then. Also, this morning the engine fired up instantly, no cranking at all.

But as other posters have written I have never seen a plug blow up like this. The one thing I can think of is somehow the plug might have been loose and each compression stroke might have loosened it enough to blow out the plug? But this is just a thought and I don't know if this is even possible? Has the ar been running OK?

[Arthur Dalton]

I had 2 out of a set of 6 where the insulators would just spin by hand in the base of the plugs.
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