Spark Plugs - Electodes falling off!

[Phil Payne]

> Yikes!!!  I changed my plugs tonight on my 93 90q.  The plugs that were in
> it were Bosch tri-electrode.  When I pulled cyl. 5 it only had one
electrode
> on it.  By the looks of it, the other two had been off for a while.  I
> didn't have time to do a compression test.  That will have to wait for the
> weekend.
>
> Now the questions...Anyone have experience with this?

Yes, seen it several times.  The engine has ingested a small hard object and
hammered it against the plug.  Drop an Aaron Bend-a-Lite or something
similar down the hole with the piston at BDC and let us know what you see.
My money is on a forest of small circular marks - 6mm or so in diameter.

The plug electrodes are hard and do serious bore damage if they stay in too
long.  Often they don't, and you only see a few marks.  I've seen plug ends
completely demolished and even pushed out of the engine.  An AAN too - but I
managed to repair the head.

Whatever does this is HARD.  An injector tip is not hard enough by an order
of magnitude or so.  The only candidate I've found in the inlet track is the
steel sleeve that locates the throttle body on the inlet manifold, but
there's no way that can get out of where it is.  It's bad enough to shift it
on the bench.  There's also a locating sleeve in the end of the ISV that's
about the right size, but every engine I've seen has had an intact ISV.

I wish I knew what caused these small object ingestions.  I've seen about
ten now, all on hydraulic lifter engines - MBs, ABYs and an AAN.

--
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803
  +49 173 6242039