Furnace flue brown goo/liquid leak

I have a goodman (gms80805cnbd) conventional natural gas furnace (installed 2017). i have brown goo/liquid dripping from inside the flue pipe to the outside of the flue pipe at a connection junction. looking at some other comments, a few options. One is the flue size, the manufacture's note says 4'' pipe. I have a 4'' to a 5'' to a 6'' line going through the roof. Can the flue pipe start at 4'' and transition to the current 6'' flue metal pipe? Should i look at the temperature settings, something else or is the flue pipe too large at 6''?
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replying to pau, Monte wrote: It would appear that the flue gases are getting to cool and some condensation is occurring due to the large flue size a flue liner would probably help.There could be other problems such as a small amount of water entering the flue as well as others. I have 40+ years in the HVAC&R field
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