What on Earth could cause a spark plug to do this?!

[Mignon]

Well I'm sorry you feel aggrieved that some comments have been negative but I think on fair reflection you might eventually admit you've really made the rod for your own back. As you now say you work on cars for a living then you should have known better than to hold a plug in mid air with the engine running and to anyone who is following this thread who maybe isn't an engine professional then don't ever do that.

However back to the actual problem. The causes of plug fouling, if we are sufficiently convinced that this is what is really happening, are well known. Rich mixture, excessive idling or very low load running so the plug never reaches proper operating temperature, contaminated or incorrect fuel, faulty or weak ignition system.

If this car has only sat around for 2 years without ever seeing the open road and has never done more than the occasional start up and idle, the fuel is maybe at least that old, the vehicle is non standard so it's not impossible there are fuel or ignition system fitment issues, non standard exhaust system perhaps causing unwanted calibration effects - that's a fair load of possibilities for plug fouling right there.I suggest you at least do a compression check to test for basic engine health, ignition and cam timing too if you can, drain and put some fresh fuel in, check or replace HT leads, try a hotter grade or at least a different make of spark plug.

[stevieturbo]

Kitchski said:

Here's a better shot of the plug. My thoughts are that it looks a bit too dirty for something that ran for 30mins, but at the same time no where near dirty enough to prevent it working completely.



By comparison, the plug that I pulled from another engine and put in this one (just to prove the theory) looked like it had been sprayed with Schutz! And it worked perfectly.

If the plug is soaked or fouled, doesnt really matter what it looks like. Throw it in the bin, even more so when you're fully aware there are problems. Again that's basic stuff. You'll go round in circles using plugs in that state

And without some live data, again, you can be chasing your tail. Information is everything. Testing wiring static with engine off isnt really going to tell much, because if there was a wiring problem like that, chances are it would be flagging up a CEL already. Not always, but in most cases.

And of course the plug is too dirty for a healthy running engine at 30 mins...but yours isnt healthy. But if there have been lots of start/stopping from cold too...that wont help as Subarus can run quite rich during warmup

[Mignon]

Kitchski said:

Here's a better shot of the plug. My thoughts are that it looks a bit too dirty for something that ran for 30mins, but at the same time no where near dirty enough to prevent it working completely.

Whereas I would say that plug is very clearly toast without a sandblasting. I assume your engine is cat controlled and therefore should be running at stoich other than at WOT. If so the plug insulators should be almost bone white regardless of how long they've run. Even a non cat engine should be at worst a light tan colour. Either there is massive over fueling, oil contamination or no good spark so the cylinders aren't firing properly. I can't see anyone remote from the situation being able to advise much further to be honest.