2008 Escalade P0455, P0172 & P0175, pulling my hair out.

[93M6Formula]

Okay I hope ya'll like to read because I want to explain this issue as best as I can.
So May last year my wife and I were driving along when all of the the SES light starts blinking, pull off the road and it's making a godawful noise... Ended up being a broken valve spring which dropped the valve and ruined the head. Me being me, I didn't want to risk simply replacing the head even though the piston and cylinder wall weren't terribly damaged.
Didn't want to pay for a new L92 or even a used one at the prices I was seeing so I said screw it and bought a used L96 6.0 to shove in it. Well that turned out to be a disaster because right I got it swapped in, started it up and was burning oil and eventually started knocking... Put 5 whole miles on it for testing purposes but this is where my CURRENT issue started... Didn't think much of it at the time but when you let it sit for 10 minutes or more and go to start it, it cranks and cranks and then finally stumbles to life. Also was absolutely GUTLESS, like reminded me of my first car with a 305, zero bottom end power.

So after this, I said screw it and rebuilt the original L92. Completely bone stock rebuild except I put DSS Forged pistons in it, that's it. Got it in and running but it still has this hot start issue. This issue did not exist one bit until the engine swap. We've been driving it ever since but dealing with this annoying issue. Eventually the 3 codes above showed up P0172 only recently, it was constantly only P0175. I've replaced the gas cap, fuel injectors, spark plugs, swapped coil packs, cleaned the MAF, plugged the EVAP solenoid port on the intake to eliminate a vacuum issue, swapped 02s, and a new throttle body. Fuel pressure reads 43.5 at idle but seems to leaks down to quicker than it should. Longterm Fuel Trims are definitely out of wack, like consistently -5 to -25 at idle for both banks, drivers side only slightly better and it has a noticeable rough idle. Lately it seems to be worse though, going as far as stumbling and almost stalling when stopped at a light. As far as cold starts, it cracks right off, almost too fast if that makes sense? Once in awhile though, sometimes when it starts to get cooler out, you can cold start it, it fires right up but begins to stumble and will stall. You can start it right back up and it will be fine. Along with all this, it's an absolute turd, I know the power it had before all this B.S. and it doesn't have it anymore EXCEPT usually the first start of the day, it will run halfway decent and seems to have power. The more you drive it and shut it off like running around town, it starts hard and seems underpowered. Not to mention the awful fuel mileage which these don't do that well already.

I don't know where to go from here, I just want my wife to have a decent running car but i've been fighting this for nearly a year, trying to work on it when I can to try something else. Even took it to my worthless GM dealer and they did nothing but cost me money and couldn't figure it out... I'm confident it's not a mechanical issue since it did this with the first replacement engine. Somewhere along the lines of the swap, something happened and I can't figure it out to save my life. Hoping I can get more answers here. Thanks

 

[diesel_dave]

Kinda sounds like the fuel pump might be on its way out? Or maybe a sticking injector that's causing the rich fuel mixture codes? You've already tried the other things I'd suggest.

 

[Fless]

I'd suggest testing the fuel pressure to see if it's within specs. Negative fuel trims indicate that the PCM is taking away fuel and perhaps there is an injector sticking or not metering properly, or not enough intake air. Is the air filter clean?

With the engine swapping and other work, has a crank correlation been done?

 

[93M6Formula]

Kinda sounds like the fuel pump might be on its way out? Or maybe a sticking injector that's causing the rich fuel mixture codes? You've already tried the other things I'd suggest.

Just replaced it last night, no change. Same with the injectors, swapped them and no change.

 

[93M6Formula]

I'd suggest testing the fuel pressure to see if it's within specs. Negative fuel trims indicate that the PCM is taking away fuel and perhaps there is an injector sticking or not metering properly, or not enough intake air. Is the air filter clean?

With the engine swapping and other work, has a crank correlation been done?

Pressure is right where it needs to be at the rail and filter is clean. I didn't do the crank relearn until recently but it didn't change anything when I did.

 

[Doubeleive]

Okay I hope ya'll like to read because I want to explain this issue as best as I can.
So May last year my wife and I were driving along when all of the the SES light starts blinking, pull off the road and it's making a godawful noise... Ended up being a broken valve spring which dropped the valve and ruined the head. Me being me, I didn't want to risk simply replacing the head even though the piston and cylinder wall weren't terribly damaged.
Didn't want to pay for a new L92 or even a used one at the prices I was seeing so I said screw it and bought a used L96 6.0 to shove in it. Well that turned out to be a disaster because right I got it swapped in, started it up and was burning oil and eventually started knocking... Put 5 whole miles on it for testing purposes but this is where my CURRENT issue started... Didn't think much of it at the time but when you let it sit for 10 minutes or more and go to start it, it cranks and cranks and then finally stumbles to life. Also was absolutely GUTLESS, like reminded me of my first car with a 305, zero bottom end power.

So after this, I said screw it and rebuilt the original L92. Completely bone stock rebuild except I put DSS Forged pistons in it, that's it. Got it in and running but it still has this hot start issue. This issue did not exist one bit until the engine swap. We've been driving it ever since but dealing with this annoying issue. Eventually the 3 codes above showed up P0172 only recently, it was constantly only P0175. I've replaced the gas cap, fuel injectors, spark plugs, swapped coil packs, cleaned the MAF, plugged the EVAP solenoid port on the intake to eliminate a vacuum issue, swapped 02s, and a new throttle body. Fuel pressure reads 43.5 at idle but seems to leaks down to quicker than it should. Longterm Fuel Trims are definitely out of wack, like consistently -5 to -25 at idle for both banks, drivers side only slightly better and it has a noticeable rough idle. Lately it seems to be worse though, going as far as stumbling and almost stalling when stopped at a light. As far as cold starts, it cracks right off, almost too fast if that makes sense? Once in awhile though, sometimes when it starts to get cooler out, you can cold start it, it fires right up but begins to stumble and will stall. You can start it right back up and it will be fine. Along with all this, it's an absolute turd, I know the power it had before all this B.S. and it doesn't have it anymore EXCEPT usually the first start of the day, it will run halfway decent and seems to have power. The more you drive it and shut it off like running around town, it starts hard and seems underpowered. Not to mention the awful fuel mileage which these don't do that well already.

I don't know where to go from here, I just want my wife to have a decent running car but i've been fighting this for nearly a year, trying to work on it when I can to try something else. Even took it to my worthless GM dealer and they did nothing but cost me money and couldn't figure it out... I'm confident it's not a mechanical issue since it did this with the first replacement engine. Somewhere along the lines of the swap, something happened and I can't figure it out to save my life. Hoping I can get more answers here. Thanks

I am no expert but just spit balling
I would check the maf readings and ensure the temp readings are correct also if not already done.
basics....air+fuel+spark

 

[Snowbound]

The pistons may be your problem. Are they the same compression as original or did you try to lower compression to use lower octane fuel?
I see it’s pulling fuel. The long term trims should be trying to get your short term trims closer to zero. Once they get to +/-25 it sets a code. I’m thinking you’re running pig rich which could be from the 6.0 swap dumping too much fuel for that engine and now your converters are clogged up making it even more rich by not flowing efficiently.
My edumicated guess is wrong compression pistons and/or clogged converters.

 

[93M6Formula]

The pistons may be your problem. Are they the same compression as original or did you try to lower compression to use lower octane fuel?
I see it’s pulling fuel. The long term trims should be trying to get your short term trims closer to zero. Once they get to +/-25 it sets a code. I’m thinking you’re running pig rich which could be from the 6.0 swap dumping too much fuel for that engine and now your converters are clogged up making it even more rich by not flowing efficiently.
My edumicated guess is wrong compression pistons and/or clogged converters.

They are nearly identical to the stock pistons as far as valve relief/CCs, like at best it might be 10.3:1 instead of 10.5:1 compression. The first 6.0 definitely was much lower and could have had an affect but I never touched a thing in the PCM. I still run 91 like it requires otherwise.
Also I did try to run it without the Y pipe and it didn't change, still started hard after sitting, the temps also seemed normal via my temp gun.

 

[Snowbound]

They are nearly identical to the stock pistons as far as valve relief/CCs, like at best it might be 10.3:1 instead of 10.5:1 compression. The first 6.0 definitely was much lower and could have had an affect but I never touched a thing in the PCM. I still run 91 like it requires otherwise.
Also I did try to run it without the Y pipe and it didn't change, still started hard after sitting, the temps also seemed normal via my temp gun.

What do the plugs look like? Running without the Y-pipe, as in no O2 sensors or cats? I’m guessing it stayed in open loop without any fueling control.
Look at the plugs and it’ll tell you if the O2 sensors are correct for pulling fuel. If they don’t look fat rich, it may be your O2 sensors.
This is a hard to say for sure without looking at the parameters while running.

 

[Fless]

...the temps also seemed normal via my temp gun.

I think what @Doubeleive indicated about temps was to check the scanner readings for the various temps sensors (IAT, coolant temp, etc.). The gun doesn't tell what the sensors are reporting.