I have a 2000 Z-71 with the 5.3L. The truck has plenty of power at take off, and at high RPM's, but when it downshifts, it has very little power. This happened fairly suddenly, and has continued. It is much less noticeable in the Tow/Haul mode, though this is likely due to the higher shift points. The truck has 106,000 miles on it. I have checked/replaced all I know to:

New Iridium spark plugs
New Plug Wires
New Ignition Coils (they were arc-ing between the dielectric can and the metal coils)
New Air Filter
More things I can't remember right now.

The truck does not, and has not ever, given a Check Engine Light or code.

Does anyone have any more ideas? I am thinking possibly fuel pump, as I have the somewhat common fuel gage irregularities, and can occaisionally hear it "sing" at idle speeds, but that doesn't make sense to me for having plenty of power at high RPMs but no power at low RPMs. Possibly fuel pressure problem? Any other ideas? Any and all ideas are appreciated!

Do you have an after market exhuast?? I know how far fetched it seems, but sometimes that can screw up your low end torque and power

Do you have an after market exhuast?? I know how far fetched it seems, but sometimes that can screw up your low end torque and power
if it was an exhaust problem it would be throughouot the entire power band.....my guess is fuel pressure problem..try the regulator first...if that doesnt fix it, then do a fuel pump...although...does your truck shudder when it down shifts during these no-power moments??? could be transmission related...

I do have aftermarket exhaust, but that had worked fine for 60,000 miles before this problem showed up.

A friend told me to check transmission; said he had a very similar problem when a vaccum hose came loose on his older model Silverado. I have crawled under there and looked, but could find no vacuum hoses on the transmission. Should there be?

I don't know if I'd call it a shudder when it downshifts, but it definitely does something. To me, it almost sounds/feels like about half the cylinders drop out and don't come back up until I get it back up over 2000 rpm. The return to power is gradual, but the loss of power is instant.

Is there a way to test the fuel pressure regulator? I see that they cost about $90, and I have already spent over $500 throwing new parts on the truck with no solution. Thanks!

there's a vacuum hose coming up off the regulator....pull it off and if fuel comes out the regulator's no good....i'd do this a couple times to check....

What about a stretched timing chain? I need to replace mine soon, I'm sure its the cause of my trucks sluggishness, as opposed to when I got it. I just put a new fuel pump on mine, that brought the high end back up a bit because the old pump was leaning out on me, but nothing down low.

Well, no fuel at the regulator, so I guess that is good. Is there a way to check for a stretched timing chain? I am sick of throwing money at this thing with no improvement! Any other ideas? Yesterday I did notice it sounded like it could be missing while accelerating (or trying to!). It sounds smooth at idle, though.

Could be something to do with a MAFS or O2 sensors. If you are getting an improper reading from those it could throw the air/fuel mixture out of whack and obviously give you problems. but IMO throwing all sorts of money at a problem is just not smart. Take it to a mechanic and have them find the problem for you and then you can fix what is actually the problem yourself.