I got a new muffler put on with an exhaust tip yesterday and this morning the inside of the tip is caked with black soot. So I think its running very rich...

Rebuilt engine, less than 2000 miles. 10w30 valvoline conventional oil.

There are no other issues that I'm aware of that would cause this. I run a Scangauge II an the idle is good. Temp runs about 190º. Runs and drives very nice. No CEL's ever. I haven't had a chance to check but it appears I'm only getting about 10mpg city driving according to the SG II. I just filled up the tank and reset the odometer so I'll confirm this when I refill it again.

Any other issues that could cause all that instant black soot? That doesnt seem normal at all.

 

Do a compression test, while you have the plugs out look for any that are not light brown - white on the insulator. Go from there.

 

Check the MAP sensor (mounted on the TB) to make sure the small elbow underneath the sensor is plugged good and tight.

 

It starts and stays in closed loop... SCII is reading the temp only as 190ish. Temp on the dash shows closer to 200. I have a 195 stat..

My guess is the coolant temp sensor??

I will check the other things mentioned too.