I've had my truck around 6 years now. From day 1 until this Spring I was averaging 17ish @ 70 mph with bouts of up to 19-20 on night runs @ 65 mph. My mileage suddenly dropped in March or April and hasn't come back up.

History: In March I believe, ran the truck REALLY hard in some sand and messed up either the torque converter or E4OD. OD light started flashing and was shifting hard from 1-2-3 and back down from 3-2-1. Threw codes for trans slip error and EBP sensor low input. Cleaned EBP tube and sensor and still threw code. Mileage decreased for the first time during this time to 12-14 mpg.

Drove for about 2-3 more months and swapped to ZF 5. ZF 5 + 4.10s=higher RPMs. Despite the higher RPMs mileage stayed about the same I'm guessing because no more tranny slip and the lower gearing of the 5 speed.

A month or so ago I pulled the 4.10s and put in 3.55 LS. Dropped my 70 mph RPMs to barely above 2,000(slightly less than with the E4OD and 4.10s) and my 65 mph RPMs to 1,800ish. My mileage has since risen to 14-15ish @ mostly 65-70 mph. Last week I installed a 6637 with no noticeable mileage gain. A few days prior to the 6637 install I made a 500 mile run and squeezed 16 mpg out of it running 65-70 and that's the best I've seen. I'm back down to 14-15 with sperratic decreases or increases with no variation in my driving-conservative with the skinny pedal, highway 65-70 mph.

Occasionally it will poot out a little grayish/blue smoke at start-up and for a few minutes afterward if I give it a "cowboy stomp" rev then it stops blowing any smoke. I don't see any discoloration in the fuel bowl. I don't have any brakes dragging(anymore at least). My tires wear great even though I could use some new tie rod ends. It does sometimes feel like something is holding it back. Not like it is lacking power but that the power is there and it isn't getting transferred to the ground(almost like the feeling of a brake dragging). Could my clutch be slipping some? At idle with the door open so I can hear better when the clutch is pushed in there's no sound other than pretty smooth engine. When I let the clutch pedal out there is a new sound almost like if you took and lightly rubbed back and forth a piece of sandpaper on a thin piece of metal. I am also smelling a slightly unusual smell. It's not quite a full blown burning smell but like something is getting too hot.

Not burning any oil and idiot temp guage doesn't get even slightly above normal. Engine sounds and feels good. Idles good. I'm going to check FPR pressure once I re-oring it.

What else? How can I check for exhaust leaks? I need to gut my cat once I can get the dang thing apart from the muffler pipe. I know it will effect economy but I really dount 2-4 mpgs. The fuel bill is killing me, especially when I know it is possible to get so much better.

 

Clean the FPR screen on the side of the fuel bowl. You'd be surprised at the difference. I'm sure the others will come up with more, but that's all I can think of right now.

 

The hard running might have screwed up the cat.

I'd start there.
Exhaust will kill mpg.

 

The up pipes if leaking can affect mileage. You will usually see some black soot around the connections at the manifolds and/or at the collector. Any exhaust leak past the turbo should not affect performance. If the cat is plugged, that could really hinder mileage and performance.

Is there any chance sand got into the engine? The boot that connects to the turbo could be cracked on the bottom side. They are known for this, causing dirt to enter the turbo.

 

another suggestion: Ran hard in sand? Have you looked at your turbo fins yet?

 

It's spotless. I cleaned it a couple months ago.
I've tried to get the stupid thing off to clear it. I guess it's just time to break out the sawzall and just get it done...
I'll check for soot too. I doubt sand got into it because the boot is sealed well.
Yeah I checked it out a couple months ago as well. It was dirty from I think the CCV nastiness but didn't appear to have any dirt/sand or knicks/chips on the fins.

I also run Diesel Kleen with each fillup. Not expecting a miracle from it but it seems to help with smoothness.

 

+1 on the cat, i think less than 15% have had luck getting the cat out unscaved LOL just cut it out! save it incase you think you might need it

 

Would leaky uppies do that? I know they'd hurt your boost and power for sure.

 

message rescinded (I did not fully read)

 

can you jack up the vehicle and spin each wheel and see (hear or feel) if there is indeed a drag?

 

I agree, even with the age and mileage the cat could be causing an exhaust restriction.

As for the FPR screen, mine was completely covered in black crude I could even tell there was a screen under it. I'm sure that was causing a fuel restrictioin leading to a heavier throttle. There a recent good short thread on fuel economy in the last week if you look.

 

Yeah, my plan is to cut it at the joint and I think I have enough slack to pull it back together after clearing it out. I'd rather have a 4" DP back straight system but I can deal with 4" cat back system for now. Supposedly they're starting to look during routine traffic stops at exhaust so I wanna have the body of it in place anyway.
I did last night actually and have a few more times in the recent past hoping that it was an easy fix like that. I don't hear anything dragging but it does feel rather tight. I put in new oil seals on all four hubs recently. I torqued the spindle nuts down to factory specs but it felt too tight. I left it anyway and it has loosened up since but still feels a little snug. Could those being too tight cause such a significant decrease.
My FPR is positively clean. I completely dismantled it probably 4-6 weeks ago and cleaned it thoroughly. Soon as I get the chance I'm gonna replace the o-rings in it and see if I need to shim it.

I'll see if I can't find that thread.

 

I guess it could make a difference if it is REAL tight. When I tightn mine, I always go by "feel" (wich I don't recommended).

 

Wheel bearing are not going to cause that drastic change in fuel economy. I would get on that cat first.

 

Unbolt the CAT from the down pipe and tie the exhaust aside temporary.
Take it for a spin and if it performs better go through the pain on removing or gutting the CAT.