I've got a 97 f150 thats getting 10-12 MPG on the highway and I think it should get 17-20. The truck has 155,000 miles and I know it was used as an oilfield truck before I bought it, which means its been on lots of dusty roads.
When I got the truck the left bank Cat was melted inside so Instead of replacing both cats I found a used one, placed it further back where where its single pipe, and extended the O2 wiring harness to where the O2 sensors would be behind the cat, as is original.
About a week ago I found that the control valve for the vapor canister was plugged up with dust, so I cleaned it and the canister out with air. It set off code 1451 the next day.
If I can't fix the problem I'm gonna sell it cause it's too expensive to drive.

Here's what else I've tried.

New Plugs
New Wires
Replaced 2 O2 sensors (No more O2 Codes)
Replaced Air Filter
DPFE Sensor
Cleaned Bypass valve on Vapor Canister
Replaced Fuel Cap
Cleaned EGR port
Cleaned VCV for EGR valve.

Also when I extended the O2 wiring harness further back in the exhaust I used solid core wire, and someone told me this might be a problem.

Thanks A Lot!!