SC . . . you need to back up to whatever set-up you had back in late May . . . you were getting 27 MPG back then!
Part of your problem here is the car is too modern. It was designed to monitor it's environment and what the driver was demanding . . . and process that info into the best possible settings for the fuel/air mixture and timing. You start tweeking and screwing with stuff on it and it gets all kinds of mixed signals. It gets confused and don't know what the heck to do.
Get a older "dumb" vehicle. They are not programmed to think. Therefore, they CAN be outsmarted.
Once again, I offer my S-10 as evidence. According to the official government web site for milage estimates (
http://www.fueleconomy.gov ) my particular truck with a 2.8 and a 5 speed manual should get 16 City, 19 Combined, and 23 Highway. I regularly get 22 to 23 combined and have hit as high as 32 on the interstate. And this is MY truck, which means it's got and extra half tone of crap in the bed BEFORE I put a load in it or set my 330 pound butt in the driver's seat. And it's got over 200,000 miles . . . and it's now got a newer rear under it that is geared a whole lot lower than the original. And just this morning when I filled up, still got 22 mpg . . . combined city/highway, leaning heavy towards city . . . and part of the small amount of highway milage was pulling a trailer behind it to boot . . . and had both windows rolled down and the tailgate up.
So what miracle of engineering got it up to the 32 mpg level? Rust. It sat for a few years before I got it and the inside of the tank rusted. Rust got in the carb. Apparently that rust has plugged the secondary's main jet. So when the progressive 2 barrel's secondary throttle plate starts to open, the fuel mixture gets really lean, really quick. To keep the thing running, I have to keep my foot off the floor. It'll still able to run 70 on the interstate, it just takes some extra time to get there. So, the thing was capable of getting this kind of milage when it left the factory, but none of 'em ever did . . . most people I've talked with never even got to the government's rated levels . . .
You see, the most effective device for improving milage on ANY vehicle rest directly between the driver's ears. Use that grey matter up there to control your foot correctly and watch the milage increase! Or, you can rig yours like mine where if you try to make it inefficient it'll just spit and sputter and die. Or, you can just let your tank rust and hope you get lucky.
