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Black soot coming out the exhaust?


NAXJA Forum User
Berthoud, CO
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.

Okay guys.

Did compression test today. Cold, dry.

1: 110
2: 110
3: 120
4: 115
5: 110
6: 121

  • Engine runs cool (new pump, radiator, thermostat)
  • A recent emissions test showed high NOx
  • Great fuel milage (18-19mpg highway)
  • Plugs look great
  • Has some piston slap on startup
So... the question is why do I have so much soot around the tip of my exhaust?
I often get a splattering of wet soot on the garage floor on startup as well.

High NOx means lean, more soot means rich, or burning oil. Getting great fuel mileage though. Compression looks pretty bad, which could mean burnt oil.
What have you done/checked/tested? What size tires?
Put a Mopar thermostat in. 83501426 with a coupla letters after that number.
You will have stable coolant temps then. Be sure to install it with the "jiggle valve" at the 12 o'clock position.