Partially burned fuel(diesel) will come out of a wet exhaust
like brown water/exhaust. Perhaps a bad injector or fouled injector, causing to much fuel
or improper spray pattern inside the cylinder, and not igniting properly on the downstroke. Or alternatively, low fuel air mixture, causing the engine
to burn rich - again excessive, unburned fuel being vaporized and vented overboard
in the wet exhaust.
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