Bosch H9DC0 Exploded!
 

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
Attachment 30773

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!
Attachment 30772

Wow :eek:

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!!!

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?

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 :P

I changed the sparkplugs on my W107 79 4450SLC
 

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?

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

I have seen the cermic center of the plug blow out and leave the metal threads in the head. The engine in queston was an Atomic 4 with probably 20 year old heavly rusted plugs. Eventually the rust got them and pop.

I'd replace all the plugs in that engine asap.

I've seen a picture in a shop manual of a spark plug captioned "severe detonation" that looked like that, but I think you would have had some indication of this beforehand, and then, why did this happen to just one?

Looking at the pictures, two things come to mind.

1. Probably a bad crimp, where the centre electrode passes through the ceramic.
2. With all the visible carbon build-up on the plug, I would do a compressions test on that cylinder,( and compare it to the others ) to see if large amounts of carbon have raised cylinder pressures very high.;)

I agree I would also, get some kind of a powerful shop
 

vac and try to suck out anything that might still be in the there. I see ACE Hardware sell various nozzle with adapter, some are I think would be thin enough to get into the plug hole. Also when you change the plugs put some anti seize compound on the treads only of the plug, if you get some on the electrode wipe if off, use a Q tip. Some folks use motor oil on the treads but I personally like anti seize. Most auto parts stores have little bags, one is enough for my 8 cylinder 450SLC.
Also, when you tighten the plugs hand tread them I use a rubber vacuum hose that is wide enought to slip down the top of the plug tighten them with that and then apply the socket wrench. Once they feel tight, turn the wrench 1/2 and extra turn just to crush the washer. You'll feel it crush.

Yea, I think a compression test would be in order
 

I have never seen a plug do this, I guess we learn something new everyday. I'll bet the compression in the cylinder is way off the others?

I had a plug do this. I had put Champions in my Supra. I had always used ND but the Champions were cheaper so I figured "what is the difference"? Well I was driving down the road and started hearing the starange sounds as described. Pulled over and found the loose wire as described. Found the plug loose and the guts gone, eaten by the engine. Later found little bits of ground up plug embedded in the catalytic. Got a bore scope and found nothing in the cylinder head but the top mashed a bit. I went back to the store where I got them and the guy says "Oh yeah, Champions do that" !!! And then the usual "common knowledge" "don't put American plugs in Japanese engines".

My only theory is it came loose and over heated. Now the oil copnsumption is up and I suspect it finished off the already leaking valve seal. Amazingly even with the high oil consumption it still passes emmissions with flying colors. Compression in that cylinder is now down a bit, maybe 10%.

Mike

Hi guys,

The car seemed to be running ok before the incident, and we didn't hear any pinging or detonation. The only part that "flew out" with the wire was the middle metal thing in the first picture. The rest of the plug, or what was left of it, was still tight. We couldn't get the spark plug connector separated from the plug wire, as it seemed to be really stuck. We improvised by unscrewing the metal thing, leaving the connector stuck to the wire, then unscrewed the end connector of a spare plug. Next, we screwed the plug into the head, then screwed the connector onto the end of the plug. Very ghetto, but it got us home before dark :)

Anyway, I'm not sure what might have caused it, but my buddy got the car towed to the shop this morning. Hopefully, the engine is still ok!