So my 90 formula is getting terrible gas mileage. I bought the car In February from a buddy of mine who drove it back and fourth from la to Santa Cruz and it got good mileage. When I bought it it had trans problems and it sat for about 4 months before I put it in the shop and I just picked it up today. Now I know the causes of bad mileage and I was planning to do a full tune up on it and I mean everything. I already did a seafoam treatment on it and the car runs smooth.
Now from what I read online is that the 5.0 tb is supposed to get 26 highway and somewhere round 14 city but mine is getting like 4 city 8 highway or less.the car is all original and has 183k and has never been reubuilt. Doing a full tune up now what else can I do to get the best mileage out of it.

 

If the mileage is that awful I'm thinking there must be something bad enough to set a check engine code. If so, get the codes off it.
Maybe it's never getting into closed loop.

 

Is it getting to operating temp? 190-200*

 

Well the check engine light hasn't been on since I got it

And the motor stays round 165-180 due to my elec fans and failsafe thermo