Rough idle

Hi I got 2000 Chevy Silverado 5.3L all suddenly driving on freeway lost power
Doesn’t rev up. When start up stays fine on 800rpm if want to drive lack of power it start supperding back firing
I did new plugs wire change new airflow meter or map sensor did not help cleaned air control speed valve .
My question is does low fuel pressure make backfire the engine. I checked the fuel pressure it’s 35psi to 39 psi it does goes up and down a lot not more then 39psi

Second thing is if timing is bad but it starts easy
I did check any vacuum leak at the intake manifold I spray arothe throutle body and other areas no idle change
So I don’t understand what causes engine not want to more then 800 rpm starts loosing power and backfire through the air filter tube

Hey Discount! Moving this thread to the repair section.

Low fuel pressure will absolutely cause popping and backfiring through the intake, mostly due to how a lean mixture will combust slower than a correct mixture. Put that together with the low power, and I think you're on track for a fuel delivery issue.

I don't have the fuel pressure spec, but 39 PSI sounds low for this engine. Disconnect the fuel pressure regulator vacuum hose and recheck. If that's the highest pressure you get, then there's some kind of pump problem. Verify no restrictions and power/ground at the pump.

A timing issue is very unlikely on this engine.