Damn for a couple hundred you might as well just buy your own wideband..

 

Well I dont know what to think. after setting my truck back to stock I made a a few highway trips around 150-200 miles and came up with 14mpg. Then my wife and I made a trip up to VA last Sunday and right before we left I changed my oil (mobil 1 synthetic 10w30 k&n oil filter) checked my tire pressure (driver front was about 2lbs low) then headed north and I could notice a difference 50 miles in. the total after a 200mile h/w trip and 75 mile city driving......17mpg!

I have no idea whats up i dont think an oil change could make that much difference

 

Fuel filter change, clean maf?

 

fuel filter is pretty new 10k ish. Ive never messed with the maf. I always get fuel at the same place because it seems to be the best in town.

Im about to re program my truck to the 87 canned tune and see what happens.

 

There are really too many variables in such a short test. Head/tailwinds, up/downhill, temperature, speed, etc. all have a HUGE effect. I personally don't think you get meaningful numbers unless you've driven several days and averaged the mileage over 1000-2000 miles of driving.

- Jack

 

I agree 100 percent. To be statistically relevant you should have at least 30 data points. In the manufacturing any capability study uses at least 30 pieces, at teh very least.

 

I understand what you all are saying but this wasn't a one week fluke in fact jackandjanet we talked about it almost two months ago. I ran at least 5 tanks of gas getting all about 300mi till the buzzer came on. now im getting over 400.

when I was a kid I put ALOT of miles on the road with my old man looking out the window of an old kenworth. And I learned the little things like tire pressure, good fuel, winds, terrain, temperature, driver habits ect.

Its not like i went up .2% i went up 3mpg

16-17mpg-canned fuel economy old exhaust
14mpg-canned fuel economy tune new exhaust
14mpg-stock tune new exhaust
17mpg-stock tune new exhaust and oil change

 

When did you install the exhaust? Also how many miles on it, and when/have the spark plugs been changed? I was about to post about my similar issue (steadily decreasing fuel mileage over 6 month period, fuel being dumped out the exhaust at WOT, occasional misfire at WOT). I have a Magnaflow SIDO Lightning-setup that I installed over a year ago, which increased my fuel mileage to around 16-18 city/18-21 highway, but around 90k miles it started doing this rich issue and my fuel mileage is now 14-16 city/15-17 highway with the lowest numbers occurring the past two months.

I've just hit 135k miles with the OEM spark plugs, so I'm almost positive that this may have some part of the issue. But this all has been on a stock tune, and I just hooked up a Livewire today to tinker with and see what happens. I'm looking at changing the plugs soon, but I'm worried of snapping them off and having to pay the stealership to do it. This may be similar to what's happening to you, or it could both be coincidental. I know I trend my mileage (~800/week) and check it with each tank, so I know for certain when and how it's been changing. The winter fuel blend has taken some out of it for sure, but my issue is that my plug gaps are huge and it's not burning all of the fuel efficiently or not at all at WOT. Hope this helps, and maybe if you figure out something else that may be the culprit it might help me too.

 

NTX- I did a complete tune up less then 10k ago plugs, wires (v6), cleaned k&n, fuel filter, new cai, e-fan, rotated tires the hole nine yards my truck has been running fine until about 2 weeks after the exhaust till i changed my oil.

As for your truck Id definitely do the plugs just spray them up then be careful. I did some on a lightning a few years back that wear a pita but with a little time we got them. how do your exhaust manifolds and donuts look? mine used to leak a little I tighten them all up when i did my tune up.

 

Mobile 1 IS "slippery" stuff, but I'd actually expect your mileage to drop with 10W30, since the OEM oil is 5W20 (I think). So, I'm totally at a loss here.

- Jack

 

I had the same problem with the 87 eco tune. I just don't use it, I leave the 87 tow in all the time.

 

I've been using the 87 eco tune and been getting low 12 to high 11's in my mpg so I changed to 91 fuel and I'm running the 91 perf tune. (no custome tunes yet) So far I'm running low 13's to high 12's mpg.

 

Mobile error

 

135K? On original sparkplugs?
Yah, that could have something to do with it.