I never had O2 sensor issues with the old engine. The truck is cooling off now.

 

They can just go bad but Id lean more towards it being a tune issue since it threw the code on both sides. Ive killed a few O2 sensors but it was always one at a time. The fact that you threw these two codes
P0171 - Fuel Trim System Lean Bank 1
P0174 - Fuel Trim System Lean Bank 2
Tells me its running pretty lean. In playing around with my tune its been leaned out quite a bit and not thrown that code.

 

Any reason why it would just do this all of a sudden after running good for the first 70 miles? It first happened when I gave it the most throttle I had so far.

 

Just heard back from Jason. He set it up for the marine injectors. I sure hope I haven't hurt my engine, running it that lean

 

I'm still confused why it just started this all of a sudden, and why it won't even pull it's own weight, and then comes out of it after a bit?

 

Who is Jason? Justin runs blackbear. Your tune is set up for marine injectors, what are you using? Check your duct tape job, any extra air getting in after the MAF will throw that code too. Hard for anyone to really help you without any logs.

 

I meant to say Justin/Black Bear. I'm using 8.1L truck injectors, as indicated in the order details. Ran it like that for 90 miles

 

Well that would be it then, the marines shoot more fuel for the give pulse width and using the smaller injectors obviously shoots less fuel in given the same pulse width. You can always try a leak down test on the new motor since you were going to get one to check out the old 5.3. Thats about the only way you can tell if you hurt it without having tuning software.

 

So you're saying there is a chance it was hurt running it that lean during light to moderate driving for the initial 90 miles?

 

I highly doubt it since you didnt put a heavy load on the engine from you the sounds of your posts. Sounds like you need to pick up EFI V2 software. Now that I actually use mine(had it for 3 years) I love it. It has been priceless in helping me get my truck lined out after installing the new engine.

You seem like a very cerebral type person, I bet youd get the hang of tuning fairly easy. Ive only really been trying to learn for a couple weeks off and on due to being busy with other stuff and Ive got a decent handle on it. Im by no means competent but Im getting there.