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Black soot coming out the exhaust?


NAXJA Forum User
What da?

Jeep is running like a champ. 17-18mpg. Spark plugs are beautiful. How is this thing making so much soot?

I do have a new O2 sensor I guess I'll toss in.
Good question. It does run cool. Goes up to 195, then tstat opens, and drops to about 170. I've tried dozens of different thermostats to no avail.
Whats the milage on the motor? Have done a compression test and a leak down test? It could be a accumulation of things.
Not sure about mileage. It was a junkyard motor I swapped in. They said 90K if I recall, and I put on about 50K since then. It does have piston slap while cold.

I just ordered a bore scope. Maybe I will check compression too. Why do you ask about compression/leakdown?
Tired motors produce lower egt's and cause the cat to not get up to operating temps for a clean burn.
Ah, that makes sense. Will check it out! If I recall, I just barely passed emissions on NOX last time as well.

Maybe this video holds a clue. I've never heard a 4.0 sound like this on startup.