Catastrauphic Plug Failure!

Hi Guys,

I need the advice of the collective.

While accelerating hard out off a roundabout and overtaking a lumbering Mondeo today my engine started making a lot of �rotary� type noises and produced smoke in the engine bay . My initial thoughts were some kind of top end failure but the obvious smoke and the fact that easing off the throttle seemed to ease the noise, I wasn�t sure. I pulled into a nearby garage and had a quick look and the engine seemed fine at tickover and on blipping the throttle. I incorrectly diagnosed that the air-con drive belt had delaminated, got caught up and produced the smoke before disappearing onto the road surface somewhere. Only later did I work out that the same belt drives the compressor and alternator! (Somebody tell me if I�m wrong)

I set off again, at reduced pace initially but then opened her up again and exactly the same noise, only this time it wasn�t going away and there seemed to be more smoke. A closer look and it was obvious that there was a compression leak or blowing manifold or similar. I had no tools with me and so couldn�t do any real exploration but I (correctly) diagnosed that there was a loss of compression on one or more cylinders. The engine seemd to be running OK, just making lots of �suck, squeeze, bang, blow� noise like a plug was missing. Worryingly there seemed to be quite a bit of oil on the plug leads . I deduced that a plug had disintegrated, although I couldn�t see below the splash plate to confirm it so, as I was only a few miles from home I got SO to bring out the tow car and tow rope. Half an hour later at home I removed the splash plate and found that the car had suffered a catastrophic plug failure (See photo HERE (Quite a big file I think!)).

I cleaned up the cam cover as best I could, got the remains of the plug out of the socket and HT cap (this wasn�t easy!), fitted a substitute plug (not quite the right heat setting but close enough to see if it would now run properly) and started it up. Bingo! No obvious long term problems.

So, these are my questions on which I seek the collective�s opinions:

Has this happened to anyone else? � I�ve heard of it but it�s the first time I�ve actually seen it.
Why did it happen? Is it just a duff plug or is there something more sinister caused it to fail?
Why did it �recover� (i.e. why wasn�t it obvious when I checked it out in the garage) before finally disintegrating?
Why the oil? � I deduce that the �suck, blow� regime either sucked or blew it from somewhere else in the engine but I can�t work out from where if the rings are intact. I removed the replacement plug to check for oiling and it seems fine so I deduce that the rings are OK.
Could ceramic fragments have damaged the rings or valves � probably isn�t tough enough is my guess � what do you think?
Should I worry or just carry on using it?

Regards and thanks



This happened to my NA MR2 at brands… Replaced the plugs and carried on for the rest of the day no problems… In fact carried on for another 3000 miles

Do a compression test as it may have bent a valve or something but IIRC it happens on the compression stroke as it due to pre detonation so ideally blows the plug out of the cylinder and doesn’t suck it in IYSWIM.

Pre det is caused by poor fuel, to hot, bad timing or a combination there of (AFAIK). As I say it happened to my MR2 once and never happened again and I didn’t go eay at all even a couple of hours after

I reckon you should be OK, if it “feels” the same then it’ll probably be OK

oooo er!!
I would’nt wish that experience on anybody.
Glad it seems to have a happy ending

I’ve now posted a photo of the plug HERE




Really sorry to hear this…

We can’t see from the photo but i presume the whole ‘tip’ of the plug is with the big piece on the left of your pic? In which case that’s possible where the oil came from? but it would have to blow past the rings, this in turn means there would have to be space which worryingly could point at damage within the cylinder bore? Was the oil only on the plug leads and under the coverplate? cause if this is yes then it can’t come from anywhere else if the coverplate was intact and screwed fully down.

Phil or Miniman would be better at this kinda conjecture.

However, if pieces of plug went down the hole while it was running I don’t think they would simply disintegrate because its pretty hard stuff. I think you should get a compression test done soon as ??

BTW how did you get all the debris out of the plug-hole? whilst trying to remove the threaded part?

I can only suggest that it recovered while you were in th egarage because it was only fractured and was still ‘sparking’ when requested by the ignition?

I can’t imagine how a plug could break like this but that’s only because i don’t have any experience of it but surely it can only happen if there is mechanical load on the outside of it? From your pic it looks like the earthed part of the tip is intact and unbent so I can’t see how something from within the cylinder bore could have hit the plug to casue this damage Have you got the plug leads tightly strapped to something such that with engine movement/vibration they cant move freely and thus put undue force onto the plug tops? BTW which plug is it?

sorry i can’t be more help but hopefully others will pass opinion…

Hi Rob,

Not a bad effort with the badge but you have found the badge of the Royal Engineers Bomb Disposal Teams, worthy and brave gentlemen but whose bomb disposal activities are restricted to ‘iron’ bombs dropped from the sky (and often beneftitting form a full ‘users manual’) and minefield clearance.

The one you want is HERE that of the Royal Logistic Corps (formerly Royal Army Ordnance Corps) Ammunition Technical Officer known colloquially by his radio appointment title of FELIX (as in the cat with 9 lives) and with responsibility for the render safe of stray munitions and terrorist devices (the latter without ‘user manuals’ of course). It is these people who have been responsible for the render safe of thousands of terrorist bombs (including all those found in NI and/or placed by the IRA and/or their cohorts)

Anyway, what has this to do with exploding spark plugs!??!


From that pic it looks pretty much what happened to my MR2 except I just had the metal casing and the ceramic part had been blown to dust

Rox, I still think this probably happened due to pre det, just one isolated instance for whatever reason, which has caused effectivly “knocking” but has also damaged the plug…

Only when mike went to remove the lead did the plug fall to peices as it was held in place by the lead.

If the compression is still Ok then there may be a few scratches on the piston and valves but personally I wouldn’t worry unless it’s really bad, we’ll all need rebuilds at some point soon

Thanks for your thoughts guys.

I’m going to book the car in for service with NW soon so I’ll ask them to check it out - I’ll run it gently in the meantime!


Major Mike

Sorry to have utilised the incorrect badge - hope I’m not scheduled to be Court Martialed!

Just thought that with your proven skill & professional expertise, you would be able to disarm the exploding plugs

PS Vinyl Veronique lurves men in uniforms, with flashy badges


PS Vinyl Veronique lurves men in uniforms, with flashy badges

Just don’t pin them on her!


PS Vinyl Veronique lurves men in uniforms, with flashy badges

Just don’t pin them on her!


What would be the point ??

She would only sigh and bend over !!

Last evening I fitted a new set of plugs and took the car for a run. It seemed fine - no funny noises and lots of power. Best of all, when I removed the plug from the affected cylinder (No 4) after the run there was not a trace of oiling, only a nice light brown colouring.

I deduce that the rapid loss of compression suked the oil past the rings and valve guides and all the ‘bits’ were blown out rather than sucked in.

All seems well.


That’s good news

That’s good news

Yeah - the car goes to Donington tomorrow and Sunday! - no doubt will see some of you there. I’ll be based on the Lotus Drivers Club stand but the car will be outside - you can all inspect my engine cover cooling ducts as well!


(get it?)