Having a really bad problem with the mileage on my truck. Its an 89 5.0 auto, on the highway the best Ive gotten in months is 12.5 MPG doing about 60 mph.

There is only one trouble code, saying the PIP is going bad. Im picking up a new one tomorrow but would that cause such a huge drop I used to get 17-18 on the highway and averaged 15 mixed.

The cats have been replaced. Plugs, cap, rotor, wires are all new. Also the fuel and air filters are both fresh. Coil, TFI module, TPS, 02 sensor are all less than 1 year old.

The idle also gets lumpy at times, no visible black smoke or soot from the tail pipe.When I pulled the old plugs they were all nice and tan in color. Im totally lost with this one

 

It sounds like your fuel source now has ethenol added.

We've been running that crap for almost 20 years in Colorado and fuel economy drops usually by the amount of ethenol added.
10% ethenol 10% loss in fuel economy
20% ethenol 20% loss in fuel economy

Fuel injected vehicles not sepcifically designened to run on E85 use an oxygen sensor to adjust the fuel mixture. When alcohols are added it tricks the fuel system into running richer and giving you poorer fuel economy.

 

Didnt think anything of that till now, reg gas now has at least 10% of that s**t in it.

 

I agree 100%, I have to drive almost 4 miles out of my way to get ethenol free fuel, otherwise it runs rough at idle an i get about the same gas mileage as you. (I have a 5.0 too)

 

what would happen if you used an o2 eliminator?