5.7 Bogs at full throttle when warm.

[Coyote25]

This is a 2000 Chevy Express 3500 with the 5.7L.  We just got it and want to use it for a trip, then resell.  It ran rough when I got it, found out one of the spark plug wires was melted on the exhaust.  Plugs, wires, and distributor look brand new, installer messed up with the one wire.  Fixed that, and now this problem.

 

When the engine is first started cold everything seems normal, idles great, revs great with throttle applied.  After about 1 minute, the idle continues to be great, but it will bog / hesitate at anything above half throttle.  If less than half throttle is applied, it gains RPM like normal. Above half, it bogs and will pretty much die if you keep your foot down.  It also spits /backfires? sometimes when it's bogging.  

 

So far I've cleaned the MAF sensor with CRC spray, new PCV, seafoam through intake and in fuel.  As I said above, all the ignition components are new, except the coil.

 

The last thing I checked was fuel pressure, it starts at 58-59 PSI when cold, and slowly drops to 51-52 within a few minutes and holds there.  I replaced the fuel filter with no change in pressure.  Spec is 60-66, but I'm seeing lots of guys online around 50 without the problem I'm having.  Could this be the problem?  Should I spend money on a fuel pump?  Or is it a sensor?

[Doug_Scott]

You may want to take a look at your exhaust to see if it has something restricting flow.  You can try putting van park, and using some common sense, floor the gas pedal and see if the engine revs cleanly, or does it simply refuse to rev past 3k to 4k? 

Fuel pressure being 10 psi low can't be good either.  Your van has throttle body with that spider injection system.  Is it all original?