asilverblazer

If the injectors aren't firing then it wouldn't continue to run after dumping fuel in the intake. In either case the only reason the injectors wouldn't be working is if they are disabled. Check all fuses for good measure but you are still probably looking for an immobilizer.

But it IS cranking, that rules out battery. I would make sure the low pressure pump is getting power when it is supposed to, a simple test light on it should confirm it is getting power.

I would recommend looking for an immobilizer of some sort. Near the OBD port or cigarette/12V ports.

When you walk up to the truck and turn it on the low pressure pump should build pressure to about 60 psi. (Unless it has been disabled by a device) The high pressure pump builds pressure over 500 psi.

I think you are confusing some terms or symptoms (no offense intended). It HAS to be cranking at some point for pouring gas in the intake to work. If I am understanding this correct: Walk up to the truck, pour gas in the intake, turn ignition key and the truck starts and runs fine. Turn off the truck with the key and then try to restart it again and nothing happens...

If the battery will crank the engine so that it can start by "pouring gas down something" then it isn't the battery. If it is only running until the gas that was poured down it is gone that means there is something preventing fuel delivery and I would suspect an immobilizer has been added. If it runs continuously after priming it by "pouring gas down something" then fuel pressure needs to be monitored. Which fuel pump was replaced? There are two, one in the fuel tank, a lower pressure pump and another on the engine, a high pressure pump. The low pressure pump may be disabled or defective and cannot prime the high pressure pump.

Sounds like a combination of dead battery and fuel delivery. Make sure the battery replacement or repossession hasn't interfered with the trucks theft deterrent system. Also, might have an aftermarket 'device' somewhere on it causing problems too. (I've been surprised how many questionable devices I've found on different peoples cars... behind panels. Most were probably a tracking device that just had a positive and negative wire, but I would'nt doubt some have greater capabilities.)

Fuel mileage is least of your problems. It most definitely is NOT. Probably in limp mode. Why? Was the engine of the correct year, make, model, type? A 2015 should not have needed a new engine. Mileage on the chassis? What is the actual code? New computer wasn't/isn't correctly programmed. The old computer should work if the new engine and associated hardware is correct. I would put it back in and see what codes (problems) it detects. Could be but I doubt it. I doubt this is connected to your engine performance and is a separate issue to be diagnosed. So you have a replacement engine in the truck with the old PCM and the only code is an O2 sensor code? There are 4 total O2 sensors, only 2 of them control engine performance. Was the correct one replaced? Next, inspect the condition of the wiring and plug connections. List any other deviations from a stock truck.

Should be in there somewhere... I know mine is set to show the current gear right now.

I think you need to fix the over heating. Sounds like it is getting too hot and miss firing (pre-detonation).

Check all grounds... confirm by using jumper cables to go straight from battery negative terminal to body, engine, frame, etc.

I'd check the rubber lines and replace those before replacing calipers.

Makes sense, since the t-stat opening and closing is the primary means of regulating the engine temperature, all other temperature readings would track with the performance of it. The temperature of the engine determines the oil temp, and a hotter engine can also transfer additional heat to the transmission.

Neither does removing the bed. Also, may not matter to you, but, I probably wouldn't buy a used truck with this kind of stuff done to it. Not criticizing or dogging on you - I'm just more of the mindset that IF the fuel pump fails, repair it properly, especially if it isn't that much harder to do it. I don't guess I understand the desire to want to do this unless absolutely necessary.

What was the issue?

At most, this truck should be on its second transmission. Why so many pre-mature failures? Your options should be to fix the cause of engine running poorly. Replacing the catalytic converters before that would be a waste. Since the truck has been seen by a mechanic - was there any kind of diagnosis or recommendation regarding why the truck is running too rich? Further, if it is indeed running this poorly, the engine should be throwing codes, so why is it not? I would suspect the previous owner has done something to prevent the truck from actually seeing how poorly it is running and thus it can't correct it, are the upstream O2 sensors intact and performing correctly? Your mechanic should be able to easily verify this and if not - find a new one. So far from what you've said I already have my doubts about this mechanics ability.

Huh, I guess that's it then. I would expect overheating symptoms (high gauge reading) of a stuck t-stat to exhibit more often than just on the highway above x mph. Make me think that maybe the aluminum block can dissipate its own heat effectively enough... given that generally speaking there aren't many fundamental differences to significantly alter the cooling system performance. I'm not arguing, just discussing the seemingly odd/specific/isolated symptoms of a stuck t-stat.

Sure, but in the cases above only at high speeds? Plenty of airflow through the radiator, that should lessen the effects of a poorly operating t-stat. (If the t-stat isn't opening, why symptoms only at high speed? It should have symptoms at other times too where engine load is high.)

You might be able to swap in a factory slider, you'd have to work it manually though unless you want to go through a bunch of challenges to get it to work powered. So to work it manually, you may need to remove the power functions so that you can operate it more freely.

Curious why the t-stat change would cure it.

If you doubt the miss fires are real, look at the crank shaft sensor. Since you say it is well maintained, may need to look a bit deeper then. Maybe an injector is starting to underperform and is at the miss fire threshold while the engine is leaning out during cruise conditions. The best scenario would be to have the scanner connected while exhibiting symptoms to monitor cylinder miss fire counts, or crank signal. Second best is to have someone with a laptop/scanner/tuner monitor and adjust while driving to find the cause. Practicality of those two options isn't best so it might be necessary to wait for the problem to become more pronounced so that you can effectively diagnose it.

uh... There is not a professional looking hole cut in the floor of your bed. You seem like your mind is already made up, lifting the bed off is really not that hard, two people can do it. Even with the access hole, getting the ring to secure the pump off would be difficult.

Its running too rich and killed the catalytic converter. You need to start diagnosing why it isn't able to correct the fuel trim to what it is supposed to be. Your mechanic says the previous owner wasn't good with electrical... sounds like something is screwed up. Why didn't the mechanic fix it? To provide further help we would need more information... any modifications to the vehicle, what all has been done to correct the issue, maintenance and vehicle history, engine size, mileage, drive train, etc.

Don't like the color, but curious about how they alter in cab noise. They should at least be easier to keep clean. Best thing I've come up with is regularly cleaning them with a pressure washer; blasting away till they run clear.