My engine ate a spark plug! (now it idles really rough)

Zoidberg · Post by **Zoidberg** » 17 Jun 2013, 19:07

I've got a 2000 S70, NA, with about 225,000km (140,000 miles)...

As I was driving home from work today, I noticed the car suddenly had less power than usual. As I came to a red light, I noticed the idle was extremely rough and the rpm gauge was going up and down about 50-100 rpms. I was afraid the car would stall, but fortunately it made it home just fine.

Although there was no "check engine" light, I got out my codereader anyway, and I found the code that corresponds to a misfire on cylinder 2. I immediately thought it was a bad coil, so I unplugged it with the engine running, and there was no change. I unplugged different coils, and they all caused a change in engine speed. I thought "aha!" but just to be sure, I swapped the "defective" coil with another one. To my dismay, the "defective" coil worked fine, and the other coil was now "dead". I decided to look at the #2 spark plug, and I was shocked by what I found: the plug was relatively normal looking, except it was missing the centre electrode!!! When I say missing, I don't mean it looks like it had been burned off or impacted somehow... I mean there's a little hole in the insulator where the electrode should be!

Now here's the weird part... I figured I just needed a new plug, so I replaced it with a NGK BKR6E, and figured it would work. It worked to a certain extent, in that the car feels like it's running on all 5 cylinders now, but the idle is still really rough.

I'm not sure what to do now... Any thoughts?

Post by **abscate** » 17 Jun 2013, 19:37

I'm a bit worried that piece of spark plug has nicked a valve.

Post by **Ozark Lee** » 17 Jun 2013, 20:09

Do a compression test across all five cylinders. I think the Auto Zone type places will rent you a compression gauge if you don't have one.

Compression gauges vary quite a bit from brand to brand and cheap to expensive so the relative numbers across all of the cylinders is important.

...Lee

Post by **BEJinFbk** » 18 Jun 2013, 08:26

That tiny piece of metal has very little mass.
It probably blew out in short order and took up
residence in the cat.

I would suggest changing ALL the plugs. OEM FTW.

m96ag · Post by **m96ag** » 18 Jun 2013, 11:14

BEJinFbk wrote:I would suggest changing ALL the plugs. OEM FTW.

+1
If you don't have access to a compression tester you can do a power balence test after you replace all the spark plugs (double check the gap). To do a power balence test start the car and remove the cover over the fuel injectors, then unplug each injector one at a time and listen to the change in engine rpm. If one cylinder is producing less power than the others the engine speed will change less when that injector is unplugged. This should give you a rough idea of what you're up against.

Zoidberg · Post by **Zoidberg** » 18 Jun 2013, 19:26

You guys are right on the money! I just had it compression tested and compression was good in all 5 cylinders (thank God!), and the injectors and ignition system checked out fine (except for one coil). Therefore, it was one dead coil that probably destroyed the plug and caused all these issues. I also replaced all 5 plugs... I replaced the dead coil, now the car purrs like a kitten (hopefully for another 225,000 km!)

Post by **mercuric** » 19 Jun 2013, 07:58

Interesting! So it was the coil that was on the failed plug?

This is interesting from a failure analysis perspective: makes me wonder if it's the coil or plug that went first. My personal guess is the plug failed first -- after it failed, spark took the shortest path to ground, which was no longer through the plug, but through a pathway inside the coil pack housing, burning a hole somewhere in the coil pack housing and allowing a continued failure as the dielectric strength of air is significantly less than that of plastic (polyethylene, a common plastic, has 7x the dielectric strength of air, for example) -- the failure could have been compounded after the internal arc did further damage to conductive parts or connections.

Have I mentioned how much I just love coil-on-plug?

Either way, glad your car is purring again and thanks for sharing the conclusion!

Post by **abscate** » 19 Jun 2013, 08:30

Ill take coil-on-plug over cap-rotor-wires any day of the week.

Simple to diagnose and much less problematic.