Different heads/taller block I suspect.

 

Interesting. I will double check that it was de-screened. It was a common F-body mod so I did it to mine. Also did the TB coolant bypass.

 

...if the maf isnt hooked up the maf table isnt used at all. You could have all 0s in there and it wouldnt make a difference if its not hooked up.

...the screen is there to straighten the airflow for the sensor so it can more accurately measure the airflow. Taking it out wont do anything for you but make it harder to tune at low throttle.

...incorrect purshrods would not explain this. They would only cause excessive valvetrain noise. LS7 lifters like between 0.08" and 0.1" preload.

Get it tuned and it will be better.

 

I did NOT do a crank relearn when I put in the tuned PCM from Nelson. Could this be the issue?

I did however have to do the VATS relearn.

 

crank relearn is NOT always necessary with a new PCM it will throw a code if needed..

 

I also have an exhaust leak. The rear manifold bolt on the driver's side broke off in the head, so there is a loud-ticking leak only when cold, it goes away after about 30 seconds.

 

Yes, get that fixed. You introducing air prior to the O2, which causes the exhaust mixture to be off thus adding more fuel.

 

Little update... replaced the plugs with TR55s gapped to 0.055". Replaced the front O2 sensors. Bought a used MAF sensor with a screen since my other one was descreened. Replaced my DIY intake with an eBay 4" CAI. Installed a Powerbond under drive pulley.

These mods yielded a increase in MPG of nothing... Around 14.5mpg on my last trip which was 95% highway, cruising average of 70mph, no AC, tires at 40psi, hauling just a T56 tranny in the bed and nothing else.

The fuel filter was replaced about 25k miles ago so I left it alone. Should I replace it again? And I am getting an SES code for the AIR system bank 2, probably a stuck check valve.

Upcoming mods are a 2/4 drop, new wheels and tires, T56 6-speed, 4.56 or 4.88 gears, and maybe a TruTrac. And I will try to fix the driver's side head exhaust leak. Hopefully these will help with the mileage.

Any other thoughts in the meantime?

 

You have a wideband?
That could tell you something. But the exhaust leak could be a big problem. If its seeing air being pulled in, the. It thinks its lean so its richening the mixture to compensate.
I fixed a driverside exhaust manifold leak cause from a broken bolt and gasket had messed up. It sounded horrible.
Retuned it and went from 14.5 to 16.5 and my daily drive is about 20 miles amd then stop and go traffic on the interstate for the next 8 miles then stop and go traffic in town. And thats with a cammed 6.0/4l80e.

Deff get it retuned and see how it does.

By the way my fans very rarely ccome on low unless the A/C is on.

 

You still have two problem areas that are more than likely the blame. You increased the size of air flow (4") which means the same amount of air is going slower. Also you have an exhaust leak before the O2 which is changing the fuel trim calibration. Until you fix the leak and retune you will still have poor mileage. If you hade a wideband installed you would see the results in AFR reading.