>Perhaps the real question, why in the world is there a
> screw on cap on one of my exhaust pipes?

Philip,
I think you're talking about the diagnostic port for an exhaust gas analyzer
-- there's another port on your cat for the oxygen sensor.  The diagnostic
port is where your local engine ninja would hook up a test rig and divine all
sorts of things about the state of your engine based on the chemical makeup
of the exhaust.

Best,
d.
1990 GL -- "Mudskipper"