V8Q Sparkplugs!

[rawaudi@frugal.com (Robert Wheeldon)]

Up to a week ago, our '90 V8Q  (80K mi) has run like a charm since we got it
in August.  Most of the miles I put on the car have been on the highway; 50
- 90.  Mostly in the middle of that range.  A week ago, I drove it to the
local car wash and when I went to return, is ran VERY poorly; like limp home
mode.  Very rich, cat smelled hot when I got home.  Acted like it was
running on 4 cyl.  After checking the connections to the coils - OK, I
isolated the problem to the cylinders connected to the right hand distributor.  

Well, I had previously purchased plug for the car but when I couldn't get
the plugs loose at 4X the install torque, had decided to let the dealer that
installed the plugs last, change them for fear of pulling the threads or
breaking a plug.  Anyway, I was now more motivated to get a plug out and
did.  Ugh!  Very cold plugs with shrouded tips and lots of black carbon!

But the story gets better.  They are NOT the same plugs as I was sold.  They
are NGK BU6EFSZ.  I go back to the parts store (specialize in performance
autos)  that sold me the plugs and asked them to help me solve the riddle.
After some investigating, they say the plug that I removed was for a 4.2L V8!??!

So, what do you think.  Did this supposedily very reputable dealer put in
the wrong plugs?  Does the auto part strore not know what they are talking
about?  Or is it possible that I have a 4.2L rather than a 3.6L engine?  How
do I tell what engine is in the car? 

 From the appearance of the plugs, they are WAY to cold and the engine
appears to be running rich.  BTW, when we got the car, the thermostat was
failed open but that was replaced about 2K miles ago.  Also, the car pass
emissions test with near zero readings although HCs were 2x the previous
emissions test but I attrubuted that to the failed thermostat. 

 Also, the former owner mentioned something about the engine being a "Mark 1
Mod 1" or something like that.  Could that mean it is a 4.2L rather than a 3.6L?

Regards,
Robert Wheeldon
'90 V8Q
Principal Engineer
Boeing Commercial Airplane