I'm sorry to keep reviving this topics, its just I have to go through a couple tanks before I can make a new post.

Anyways, I have went through and done a couple of the previously mentioned suggestions and I have some nice results. After replacing the O2 sensor and switching to Top Tier gas I was able to get about 12.2MPG per tank. I then went and replaced the Fram air filter I had in there with a Spectre Performance air filter and (against the advice of you guys) replaced the MAP sensor. I switched to Chevron gas and am seeing 12.6MPG per tank(which is well within the margin of error), so either these changes didn't make any difference (which seems unlikely) or there is still something else bottlenecking the truck.

Also, I though I would just add a bit of information regarding my driving style, the vast majority of my driving it highway miles and I typically just set the cruise control to about 68MPH when driving. The area is a bit hilly, so it may make it a bit worse, but 12.5MPG still seems low for all highway miles.

I currently have a tank of Shell gas in that I am going to burn through to see if maybe the ECU needed more time to re-calibrate to the new MAP sensor, but I'm not too hopeful. Following that I think I may try gutting/replacing the cat, new plugs and wires, and a new cap and rotor.

Any other suggestions would be greatly appreciated.

 

If you have a pressure gauge of some sort, drill a small hole in the exhaust right before the cat, and see what pressure is at idle. Up to 2 psi is 'acceptable'..... anything beyond that, and you need to find the restriction. (probably the cat itself.)

But yeah, on its best day, I was able to squeak 18mpg out of MY truck....

What RPM are you running at 68 mph?

 

I checked the pressure in the past and I had a gauge threaded into the front o2 sensor port and I was getting effectively 0PSI, the needle bounced +/- 0.1PSI with the engine. And, at 68MPH, I am sitting at about 1900 RPM.

Another potentially useful piece of information, I replaced the fuel pump about 20k miles ago, is it possible that the fuel pump (or my install job) may be defective? It was branded as "Autobest" and I doubt it was a reputable brand.

 

When you say you run Seafoam every oil change, do you mean the carbon cleaning through the brake booster or just dump it in the tank?

Doing a carbon cleaning can help immensely with idle, mpg, power, etc.

Also check the wheels to see if one is extra hot caused by dragging brakes.

Slowing down to 65 or under will get you another 1mpg but I ran 68 on my road trip and got 16 so maybe there's something else. Again, opening up exhaust helps a ton.

 

I do it down the brake booster, I noticed a rather significant improvement in responsiveness the first couple times I did it, but it seems to have diminished. That dragging brakes is a good idea, I'm pretty sure that my drums are out of true and that rotors aren't in the best shape either (I notice a pulse slight pulse when I brake) so they could be causing some drag. I think that the rotor on the passenger side is beyond repair, so I should replace it regardless. I also read about swapping the calipers and wheel cylinders with those in the 3/4 ton trucks, I may look into doing that when I replace the rotors.

Also, would something like a 180 tstat help mileage?

 

No, the 180 stat may actually DECREASE your mileage....

The numbers look good for speed vs. RPM...... Still and all, you should be able to do better. May not hurt to jack it up, and spin things by hand, see if anything is obvious. I doubt that the front axle staying locked would have that much of an affect on fuel mileage.

 

I didn't really notice any issues with the brakes causing the tires to bind, but I really do need to replace the rotor and pads. I think just doing a basic tune up would help bring me to a more acceptable range, and considering I should be passing 200k by next weekend, I think it is a great time to do it. Here is what I am thinking about doing:

Plugs, cap, rotor, wires (not sure if the wires need replacing, they are Bosch)
Seafoam cleaning, oil change
Transmission filter change, installing a remote filter for transmission
Brake pads and rotors
I may also gut the cat, install new headers, and change the muffler

 

Sure can't hurt..... Reset the PCM as well, and let it re-learn everything.... maybe that will help.

 

I'll try to get it all done by next weekend and post a status update shortly after. Also, I got one of those Bluetooth OBDII scanners so I could perhaps get a quicker estimation of my mileage, I still haven't got it entirely setup, and I'm not sure how accurate I could get it (last trip it said I averaged 52MPG), but I figure it couldn't hurt.

 

if only it were telling the truth......