Abnormally Terrible Gas Mileage

Unbolt the flange between the exhaust pipe and the cat. Should start running better right away if the cat is plugged.

You can't run a test pipe unless you want to start dealing with O2 senser simulators. With no cat you will soon throw the "exhaust inefficiancy" code. I forget the number. This puts your truck into "open loop" mode. The best way to describe it is the computer leaves the engine management in default, wich dumps in more fuel and your milage goes in the shitter.

 

Or if you don't want to go to all the trouble connect a vacuum gauge to the engine give it some gas and if the vacuum is for s$%^ replace the cat. If the cat is plugged enough to waste fuel it will have no guts at all so if it has sufficient power it ain't the cat.

 

With no CEL, I would not suspect a cat or an O2 sensor.
Something is causing it to dump fuel, but it is apparently not bad enough to trigger a CEL.

Does your oil smell like fuel? If you are running that rich, it likely would.
Same with the exhaust. Any raw fuel smell or smoke?

You mentioned iridium plugs... do you have the 4.0 or the 2.7? If you have the 4.0, make sure those plugs cross-reference correctly... factory spec for the 4.0 is copper and I don't know if the part numbers are the same for the 2.7.

Finally, how is it running? If fuel consumption is THAT bad, it should be running like crap.

If none of the above, and you aren't letting it idle for a long time to warm up, I'd put a trail-cam on your parking space... someone might be stealing your fuel.
They most likely can't siphon it, but at least one member has found a fuel hose RTV'd into his tank, capped off, and tucked up in the wheel well.
Thief would come by every few nights and pull a few gallons, tucking the hose back up for next time.

 

Made an interesting observation on my 2014 with automatic tranny. If you have big feet like me, it is really easy to have your foot on the accelerator and catch the brake as well. Thus, driving with the brake on. That would really screw up your mileage. I wonder how many fellas with gas mileage issues are riding the brake and not realizing it?

 

haha that could be possible! I drive with my work boots on and theyre not tiny, but I don't think that's it.

 

I read up a little on plugs, I have the 4.0, and I believe you can use both the iridiums and or coppers. That's not a bad call, but I was having this issue before I switched the plugs thinking maybe that was the solution.

Doesn't smell like it's running rich either. I will have to check the oil tho. What would it mean if my oil were to smell like gas? leak?

 

Iridium itself is not a problem. It doesn't help, but it won't hurt, as long as it is the proper plug (proper reach and heat range).
Iridium simply allows 120k change intervals.

But heat range is important. If the plug is too hot, it will lead to detonation, which will cause the knock sensor to retard the timing, which hurts economy and power.
If the plug is too cold, it will cause an incomplete burn.

If the engine is running rich enough to pull the MPG down by 50%, there's going to be a lot of excess fuel in the cylinders.
This is very bad. It washes the oil from the cylinder walls, increasing wear, and it contaminates the oil.... circulated with the oil, and gasoline is not a good lubricant.

 

If the plugs have 150,000 miles on them they still won't cause a 50% drop in mileage. Snow tires, very cold weather and winter fuel costs me about 3MPG drops from 20 to 17. Using that much fuel is going to throw a code if nothing else an O2 sensor for over rich mix it will over heat the cat and belch black smoke. I think you need to get new batteries for your calculator.

 

Here's a smaller idea that wasn't mentioned worth 2 mpg,,,, defroster. When you change the interior venting to defroster, it kicks on the A/C compressor as well. That will result in some loss of mpg but not to the point of 9mpg for sure.

 

Not even a 2mpg drop.
I don't see a 2mpg difference between winter and summer... summer I'll frequently run the AC even on the way to work in the morning.
Modern AC systems are extremely efficient... from a fuel consumption standpoint, but they will no longer cause frostbite like the old R12 systems could.

 

I read this a day or more ago, but didn't have any worthwhile ideas to post. The big drop without noticeable, significant problem I found puzzling. Somebody stealing fuel never occurred to me, but makes a lot of sense.