My brother-n-law has a 2004 Tahoe with the 5.3 flex fuel engine.
His LTFTs are maxed at 25%.
I did the simple things first, I cleaned the MAF, and checked fuel pressure and found it idles at 40 psi, 38 psi revving, and 50 psi with the vacuum line pulled off of the fuel pressure regulator.
With the fuel gauge still hooked up, I clamped the return line and the pressure jumps to 80 psi.
I pulled the regulator off and inspected it, I found that it is bypassing so we replaced it with a new regulator.
Before I installed the new one, I checked it for bypassing and it does not, its perfectly fine.
I installed the new regulator, and still low pressure, 40, 38, and 50.
And when I clamp the return line, it jumps to 80, and when we shut off the engine, the pressure holds at 40 psi. It does not bleed off.

Is the fuel pump bad?

Iv'e read several post and they seem to say it should be between 58 psi and 62 psi, but most also say that they don't get a change when they clamp off the return line.
We do, it jumps to 80 psi.

Any help will be appreciated.
Thank you.

 

Forgot to mention, we replaced the fuel filter too.

Also, I didn't have any starter fluid on me to check the intake and gaskets for leaks.

I guess my next thing to do is to get some starter fluid and check for vacuum leaks.

 

Sounds like it could be the pump. Regulators do not break that often.

 

Update...

Ok, I fixed the problem.
It was the intake gaskets.

I decided to do a test drive with the scanner, I saw that the LTFTs would go normal under acceleration or a load.
I figured then, that it was a vacuum leak.

When I got back, I left the engine running and sprayed starter fluid around the intake, instantly the LTFTs went negative.

I replaced the intake gaskets and everything is running fine now.
LTFTs are around zero again.

 

Intake leaks are usually coupled with a high idle speed. Was it ideling high?

 

No, it was idling around 600.
I did notice the MAP was at 35 Kpa instead of 33 - 34.
And the throttle was closed at 1%

 

Your fuel pressure still isn't right, and 1 kpa different in map reading doesn't mean ****. Under a load fuel trims are supposed to go 0 as that is open loop.

 

Yep I agree, fuel pressure isn't right. Low pressure causes maxed trims, I've seen it personally on multiple occasions. Motor can otherwise drive fairly normally it will just be down on power but even that may not be extremely obvious.