TexasMustang

I bought my 06 5.4 Lariat 4x2 about 3 years ago with 115k on it. When I first bought it I was averaging around 15mpg. That slowly dropped to around 13 mpg and I replaced the plugs, coils (with new motorcraft), and fuel filter. Also put in an k&N panel air filter and recently cleaned the MAF sensor. It didn't make any difference. The mileage has continued to gradually fall and now I'm averaging around 11 mpg. Truck now has just shy of 160k on it.

The truck runs good. Smooth idle, no CEL, a little tick at start up but it goes away quickly. It does feel like the truck has less power then it did when I first got it, but it isn't a dramatic change and could be in my head.....

Any suggestions on what to check next? Bad 02 sensor or cats?

MGD

Howdy.

1. Yer front O2's may be getting lazy - yer due fer replacement @ yer mileage in any case.

2. Have yer cats checked fer health.

3. Clean yer TB.

4. Ensure you have no stored pending codes (no CEL) are present. Low-grade misfires are a possibility (which eventually toast the cats).

5. Yer right - the K&N does not help - no perf gains, and the very real possibility of MAF contamination.

Good luck
MGD

Nosajb15af

Wow, this is a loaded question. So many answers. Fuel quality, driving habits, bearings, brake drag, O2 sensors, MAF, compression, cats, tires, 4x4 drive train stuck rotating, cam phasers, plugs, oil wt, weak spark, possibilities are endless.

PyramidsRs

Clean your throttle body as well.

Double check your plugs, I noticed a difference almost immediately after doing mine.

Also check the fuel control module thingy on the frame by your spare tire, that could somewhat affect fuel economy as far as I know if it's starting to corrode.

Mod (Ret.)

To the OP: did you change to a larger rim/tire size? Rear gear change? This too can have an effect on fuel economy. Accessories that add weight are also fuel economy lowering issues.

TexasMustang

The truck is all stock including tires and wheels. Roads and driving conditions haven't changed either and I still by my gas at the same station so those variables should all be ruled out.

There are no error codes stored in computer.

I replaced the front 02 sensors yesterday and cleaned the throttle body. The computer is still showing 11.2mpg..... I usually check my mileage by hand and it is usually very close to the computer so I'm guessing this is still accurate. I'll know after I use a full tank.

I also used a laser thermometer to check the cats. both cats measured about 600f at the front and 450f at the rear. This is backwards isn't it? Front should be cooler then rear for the cats? Should I pay a shop to pressure test them? Or is there anything else I can do from home to test them? Or is the temperature proof they are bad and I should replace (maybe remove).

elricfate

You know that computer is based on a 300 mile average, right? It's going to take awhile before it knows what's up. Even if you do a reset on it and force it into a recalc it's not going to be entirely accurate. I get about 15 hwy and 11-9 city, depending on how terribly I drive.