90cherokeeguy

yesterday i went to where my brothers 99 is parked i knoticed there was a black spot on the white driveway underneath the tailpipe so i cranked the motor over and was told there was stuff shooting out so i grabed a pice of card board and this shot out

also the plugs are fouled in this stuff whats causing it and how do i fix it

BuckB91XJ

Black smoke and crud out the tailpipe means an overly rich condition....or it could be contaminated fuel. Can you tell us about anything out of the ordinary that was done to the jeep over the days between when it last ran good to when the black crud started coming out the tailpipe?

90cherokeeguy

it got a new cat and muffler put on last week

BuckB91XJ

Well, if the Jeep was run any amount of time with a plugged cat or muffler then there would be some soot built up in the exhaust system from the motor not beinmg able to breathe good. The black crud should have cleared up after running it for a few minutes and getting it up to normal oprating temperature if this was the cause. Do you know if a new O2 sensor was installed when the exhaust work was done? I believe 99 XJs have an O2 at the cat. Hopefully your exhaust shop didn't just remove it.

Bustedback

That stuff simply means your new cat is working. Water is a byproduct from the chemical reaction that takes place in the cat convertor, that hot water is now flushing all of the soot from the exhaust tubing and muffler. Now the question is, what caused the original cat to go bad? Cat convertors do not go bad by themselves, they get damaged and ruined by other components that fail and cause the air/fuel mixture to be overly rich.

90cherokeeguy

i instaled the cat my self i put a o2 sensor after the cat the put not the other one

BuckB91XJ

That could very well be your problem. The newer XJs that come form the OEM with the upstream and downstream O2 sensors...the computer is designed to need inputs from both to set the air/fuel mixture properly.