FE getting progressively worse over time...troubleshooting help appreciated.
 

Well, I just updated my fuel log for my F-150...and I've hit a new all-time low, despite no change (or even modest improvment) in the "nut behind the wheel" skills.

13.3 MPG!:mad:

That's WITH EOC, DFCO-aware descents, timing red lights and not exceeding the PSL on highways. For the first time in ~20 years (even before I knew what hypermiling was), I'm UNDER EPA!

In the search for a mechanical problem, I've checked for vacuum leaks, read the existing plugs, performed a compression test, gapped and installed new plugs, and topped off all fluids. (Oh and I'm keeping the tires at 40 PSI.)

All tests were normal, zero carbon fouling, and 170+ PSI on all six cylinders.

I'm at a loss. I'm beginning to suspect a plugged CAT, simply because it would explain a steady decrease in economy (plus the fact that I lose power--seemingly--at WOT, and get an odd "pfft, pfft" exhaust sound: kind of like a headgasket breach, only less so.)

How would I test for this? I mean, besides just putting in a test pipe section for a couple of tanks? (Not that I'm terribly averse to that as a temporary diagnostic tool, mind you.)

Any other areas to look? Like checking timing?

P.S. I have won the missus on the idea of motorcycling:D, so now the only use the truck gets is work-related, trips in averse weather, or for buying more stuff than would fit in a backpack. "Using a truck as a truck" would explain SOME drop, but I would still expect the I-6, MT, 2WD w/ 3.08 final drive to do quite a bit better...

O2 sensor, 94 explorer went from 19 to 11 in a day, maybe your's it failing slowly. How's the tail pipe, any soot? Mine was black by the time I got home cause it had to happen 400 miles from home on a Sunday.

It could be an o2 sensor or a plugged cat, both can be problems on f150s. My bros old 96 f150 had the cats pretty badly plugged to were the lines were blowing off the egr valve i believe. and beings its age and o2 sensors normally being overlooked as problems the precat sensor affects your fuel ratio so if its in needing of being replaced it could be your problem as well

Try the 02 sensor then if it doesn't improve the mileage pull the cat and inspect it for blockage. Mine did 20 highway but that was the best mileage it got and it was all highway, over 800 miles worth in less than 24 hours at 65-70. Mine had the 5 speed and a 2.73 rear ratio. I didn't even realize it was in 4th gear for 10 miles coming back at night.

regards
Mech

I remember the owners manual in mine stated that Ford would replace the PCV valve for free at 60 k miles. I would at least check to see if it was clear and the hose as well.

regards
Mech

Cooler weather will drop your fuel economy.

How the brakes, maybe one's dragging getting worst & worst.

Update: cats removed, test pipe in place. Will update.

Don't think it's a bad O2 sensor, as plugs were as carbon-free as I've seen. Will test, though, along with ignition timing.

PCV replaced, brakes observed not to be dragging.

I'm beginning to suspect "curb crawling" is a lot worse for FE than I counted on. Figured on 12 for my scrapping forays; now thinking 7-9 is more likely...;-(

Us insight owners will see a mpg drop for no reason when the 12 volt battery needs replacing. Beyong the mpg drop you get random idiot lights on the dash to needing a jumpo to start the car.

I take it that your F150 has a carburetor.

My neighbor has an older F150 and it's definitely carbureted because it "burbles" when it's idling.

Kind of chuffing/coughing sound out the tailpipe, and I suspect that it's probably running on the rich side.

My 25 year old motorcycle also has carburetors (4 of them), and I have to be very careful with the air-idle-screws to get it to idle correctly.

If it's the year F150 that I'm thinking of, it could definitely be a carburetor issue. Just a thought.

Jim.