A4 back on the road

[Phil Payne]

> I don't understand how a particle could migrate from one cylinder to
> another.  Can anyone explain?

No.  But it happens.

The usual object seems to be a ring of pretty tough material, about 6mm in diameter.  I've
only ever found small pieces of debris nestling in the bores above the top piston ring.  I've
seen four engines in which one piece of debris has obviously migrated between the cylinders.

If you take the I5 10V throttle body off, you'll see a short pressed-in steel tube that acts
as a locator between the throttle body and the inlet manifold.  This thing would be an ideal
candidate for the damage you often see - however, I've always found it in place..  The last
time it happened we stripped the engine EXTREMELY carefully looking for a candidate.  There's
no way anything big enough would pass through the intercooler, so the source is inboard of
that.  I've searched high and low, and only just tonight while going to sleep have I started
to wonder whether it might be part of the spark plug.  How is the central electrode held in on
the three-prong plugs - is there a ring held in by the prongs?  If the prongs are removed,
does a steel ring drop out?

I'd be interested to know what the debris marks were like in this A4.

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