How's your oil consumption? I had a black ring on my exuast pipe and always assumed it was due to the standard over eating of oil in my 99 Rodeo. My O2 sensor was good and fuel system cleaned regularily and just figured it was carbon from the burning oil...May be your cause.

"WE ARE ALL A PRODUCT OF OUR OWN DECISIONS"

92 Mitsubishi Pajero XR 2.5 TD (SOLD and feeling lost without it)
OME Suspension
American Racing steel Wheels with
Dick Cepek FCII Tires