For years ... years ... my 22yr old Jeep with only just over 61K on the clock, has gotten terrible ... terrible ... gas mileage and the automatic transmission was surging and shifting hard into some higher gears.
I couldn't figure out what was going on. I swapped out sparkplugs, changed oil and air filter. Even drained and installed new transmission fluid and transmission filter. Didn't help.
A couple of days ago, the Jeep finally threw a Check Engine Light (CEL) on the dashboard. Turned out the fault code was a P0455 ...
"P0455 is an OBD-II diagnostic trouble code that indicates a major leak in the evaporative emission control (EVAP) system, which is responsible for preventing gasoline vapors from escaping into the atmosphere. Common causes include a loose or damaged fuel cap, damaged hoses, or other issues within the EVAP system."
So I crawled around all over the vehicle and FINALLY located a ruptured rubber hose near the gas tank and replaced it.
Today, the Jeep is running the best it has in I can't remember when ... years ... and the automatic transmission shifting is no longer an issue what so ever. The Jeep runs and shifts as good or better than it EVER has. Not only that but my miles per gallon has jumped up 4miles more to the gallon and now getting the gas mileage it did 22yrs ago when I drove off the Jeep dealer's lot with a brand spanking new Jeep.
My Jeep is happy ... I'm thrilled. Even have some left over rubber hose should another need arise.
Still NO idea how the EVAP system affects the automatic transmission but just glad replacement of a ruptured rubber hose fixed it.