Hey folks! I'm getting horrible fuel economy! No CEL 302, AOD, 3.08 gears with 31.50x10 tires, I'm getting 7.8mpgs. It had an upper intake leak so I resealed it aND fixed all the vacuum lines and I went from 4.3mpgs to the 7.8. I'm getting black soot on the tail pipe and it's definitely burning rich. It's an 89, if i cant figure it out I'm going to slap a carb on it!

 

check the codes even if your not getting a CEL. start with checking the coolant temp and O2 sensor. check the ground for the o2 also. make sure the fuel pressure reg isnt leaking inside. check the map sensor too.

 

the black soot is you definitive sign you are running extremely rich. Don't count on your engine to throw codes for every thing. Like Redark said, check the codes anyway but it could be anything from your injectors, to the fuel pressure regulator or anything else. With your configuration, AOD, and such high gears and tall tires your MPG's won't be the best anyway. Your RPMs are extremely low and while this may seem great on the interstate goin 75mph. Driving around town and what not you will spend all your time accelerating and not at actual coasting speed.