Had a curious event happen today. The family went out to Sweet Tomatoes
(salad bar buffet restaurant chain)
today, my 20 year-old daughter drove the 2000 Toyota Siena van. While
getting on the highway, she floored
it for about 10-15 seconds to be able to merge in front of a monster RV
(greyhoud-bus class). She commented
about the car "shaking" right after that. I didn't notice it at first,
but soon it was more pronounced, felt like
an intermittent miss. Later, it got worse, I stopped by an auto parts
store on the way back home and bought
a set of spark plugs, taking a wild stab at what it might be, and
knowing they hadn't been changed in forever.

The rear plugs are a total bear on this car, but the internet helped me
find out you take the cowl panel with the
windshield wiper mechanism out to get access to the rear bank of the
engine. One plug of the rear bank
had the ground electrode bent all the way against the center electrode.
I don't THINK I manhandled it enough
to do that getting the plug out of the well and past the intake
manifold, but I can't be absolutely sure of that.
Obviously, the plug wasn't like that before the misbehavior, could a
severe knock do that to a plug? Could a chunk
of carbon have come off the piston and crunched the plug electrode? It
was bent, but not totally smashed.

This car has no distributor, and has 3 spark coils. So, each coil
serves two cylinders. What happens if one plug
is shorted? Does that short out the other plug, too? Was the other
cylinder missing intermittently, as the coil
barely had enough current to make a weak spark with one plug shorted?
The shorted plug was on the branch
with the long plug wire, the coils are on top of the front bank plugs.

Got all the plugs back in, started it up and it ran smoothly. I didn't
drive it yet, but before the work it was
running so badly I thought it might leave us stranded. Unless it needs
the leaner mixture for a warmed-up engine
to misbehave, it sure feels like it is fixed. The check engine light
came on as we were driving home,
I was waiting for that to happen.

Anyway, has anyone had an experience like this, where there was very
obvious ignition miss immediately after a
full throttle acceleration? I've had plenty of dirty spark plugs and
bad plug wires over the years, and they generally
slowly got worse. I've never had a miss develop suddenly while
driving. All the old plugs looked quite normal,
although the gaps were all worn a bit.

Jon