Major Drop in Mileage, but no loss of power.. Totally stumped
 

Hey guys, here is a question for ya. I know this subject as been hashed out several times on here but I am at a total loss. Here is my situation. I have an 07, I5, 60000 miles on it. CAI, magnaflow dual exhaust, PCM4less, K&N filter, and all the goodies. I run 315s and up until a few months ago was squeezing about 300-325 miles out of a tank of gas. Getting about 15-16 Mpg.. I was ok with that cause the truck has decent power for what it is..

At 55000 miles, I did all fluids including, trans, transfer, rear and front diffs, new AC Delco Factory plugs, new O2 sensors, new Optima Red top, cleaned the throttle body, cleaned the MAF with MAF cleaner. I recently noticed that temp was way to low and replaced the thermostat, which brought the needle back up to the half way point, after the new thermostat, the radiator cracked across the top, so that was replaced along with a new waterpump for good meausure. I have done everything I can think of. I am good mechanic and have a buddy of mine who works as a major shop here in town help me with all the work. I keep the rig in tip top shape, Have never had any issues with it.

I am beginning to have a love hate relationship with her, Every tank of gas I am loosing about 10 miles off the the odometer. At first I was getting about 300 out of tank, then dropped to about 290, then 280 and so on.. Now I am down to about 200 or less per tank.

I know the ol winter fuel facts, but I have a station that has 92 octane no ethynol added fuel, it does get better mileage using the fuel but its also a $1 more a gallon compared to super with the eth added.

I have had several shops look at her, drive her, I am getting no codes, no misfires, no power loss or any symptoms of anything wrong with other then its getting more and more expensive cause I am filling up all the time. Right now I am averaging about 10 miles to gallon on a good day. Every paid mechanic that has looked at and diagnosed her is stumpped!!!

There is no compression issues, no head gasket issues, no misfires, no Codes on the OBDII, nothing major going on.. So help, please!!! Anyone have an issue like this? I am thinking about starting over and doing another PCMtune but its spendy..

TAINTER, or fellas you have any insight? Cause if this does not get better I am gonna dump her and find something else to buy, I dont want to do that cause I love the truck, the way it drives and its perfect rig being a single father.

Do you always get gas at the same location?

Go stupid with it, knit pick...
1.) tire pressure them same on all 4 tires?
2.) fuel system gone over?
3.) intake system cleaned thoroughly including air filter?
4.) how is she idling? Excessive play in the needle while driving? i.e. bouncing from 1500 to 3000 rpms?
5.) that cargo box been on all along?

Anything else you can think of?

Do you have an extra pcm to swap out to see if the tune is the culprit? O2 sensors being in open will use a lot more fuel as well, Generally they will throw a code but not always.

Are you saying you are running 92 Octane? Unless you had the PCM tuned for high octane that is not going to do anything for you.

Have you always had a roof rack and carrier?

Where are you located?

Check the brakes, corroded calipers = brake drag.... you know what I mean.

When you changed the trans fluid, how did the old stuff look?

Sounds like your fuel gauge is lying to you.
 

Might want to check it out.

my thought says the tune is off now. changing fuel grades isnt the issue. pull the tuner off and see how it does for two tanks. that or pull it off and send it back to the manufacturer for a diagnostic and/or software upgrade.

and ya, how long has that aerodrag box been on there? I just came from a ~1,500 mile trip to Az and back with a cargo box on top (xmas gifts). mileage took a big hit especially with the high winds I hit in the carlsbad area of texas/new mexico.

Sorry, the only issues I can think of are bad O2 sensor, torque converter is always locked up or some kind drag on the rig (brakes).

PCMforless tunes your PCM, so you can't remove it and continue to run the truck. You could swap it for another with the VIN for that truck in it like talusfan suggested.

However, you bring up a good point.... did anyone of those shops that checked it out scan or flash the PCM?