2000 silverado 2500 6.0 random lack of power

Hey I’ve had this truck since 2009, it’s a 2000 silverado 2500 (nonHD) extended cab long bed 4X4 with the cast iron head LQ4 (6.0) and 4L80E trans.
It has almost all the options of the day…

I bought it with 77K it now has 138K and has had this problem since the day I got it…
When accelerating it intermittently has no power from a stop. It has really high gears (like 3.42 or 3.73) as opposed to our farm truck which is an 05 2500HD which has 4.10’s…

What happens is when I take off from stop, it will be a total dog off the line it doesn’t matter if the tank is full or empty, truck is hot or cold, part throttle or WOT. Regardless it has no freaking acceleration. That is until you get to just before the shift point then the power comes back and it puts you in the back of your seat just like a 6.0 should. Then the truck shifts into second and has the normal amount of power.

I have had several local mechanics try to fix it, it has been given a tune up plugs wires filters etc. It has had the TB and MAF off and cleaned. Over the years the fuel pump and sending unit replaced etc.
Over the years I have replaced the O2 sensors (with ac delco) both cats, IAC TPS crank positioning sensor, EGR and possibly others…

I have put a fuel pressure gauge on it but it never seems to have a problem when you have time to mess with it…

Possible other theories include but are not limited to: Fuel pressure regulator? some intermittent vacuum leak? partially collapsed lifter?

NO CEL, it runs smoothly though lately (last month or two) the idle is all over the place… tends to idle high…
It has had a slight knock since I’ve owned it… I had a theory that maybe the knock sensors were pulling power, but wouldn’t that trigger a CEL?

Has anybody hooked up a recording scanner to the OBD 2 port to watch what happens when the truck loses power? If not, you really need to do, or have this done.

Tech 2’s, laptop and smartphone systems from many sources can record various OBD 2 parameters that you can analyze later to pinpoint the trouble. Torque, is free app but the fancy full function version is $5 for Android or iPhones and works with a Bluetooth OBD 2 reader tool available from Amazon for $10. So $15 gets you a data tool that can log the sensors so you can read the knock sensor to see if it recording engine knock. No, a knock sensor would not trigger a CEL, it just pulls spark back until the knock stops.

You are a really patient person. I would have long ago traded a truck that had an un-fixed issue like this.

On these engines, the knock sensor harnesses get brittle and corroded

The knock sensors and harnesses are in the valley, and water has nowhere to go

Ae you quite sure your intake gaskets aren’t leaking? If you still have the original orange, they’re screaming for replacement

Are your fuel trims correct?

Does the engine start normally?

No clue on fuel trims, etc. I haven’t touched intake gasket, but I’ve sprayed water all over engine (vortech cover removed) and no change in idle, and I’ve tried it lightly with starter fluid (when cold) but I’m not a fan of that method…

I wouldn’t say I’m patient… I just seem to end up throwing a couple hundred $$ at it a year and get sick of dealing with it… It gets me around just fine… Just need to beat on it a little and it goes away… lol I keep thinking if the thing would just blow up i might figure out what was causing it :wink: But nothing ever breaks on this truck it just runs without much power on occasion… lol
I can’t replace it till it’s dead or fixed… If it is dead I can get something newer, but if it isn’t fixed I won’t get any $$ for it… lol and the damned chevy just keeps going… Hell this thing has been through two floods (I’m also a firefighter) and has forded water almost 2’ deep at times and never had a hickup… (other than the first flood where I ripped the front axle actuator off on a boulder getting to a flooded person on a washed out road) but a new actuator and a change of axle/Xfer case fluids later and all is still as mediocre as it was before… :slight_smile:

Yes engine starts normally.
What valley are you talking about the knock sensors being in?

I think it’s a leak in the intake manifold gasket.

how would i test that? and why so intermittent?

You test it with an evap/smoke machine. You could also use carb cleaner while watching fuel trims with a scan tool. But the smoke machine is far more efficient

You could also blow cigarette smoke into the plenum, and see where it comes out, but I think you’ll be huffing and puffing

Another idea . . . spray carb cleaner at the intake gaskets while the engine is idling, and try to acoustically determine if the idle has improved

The valley is the area between your 2 cylinder heads

Since you have a V engine, this is literally a valley

Another thought . . . your engine is 100% notorious for having sheared off exhaust manifold bolts. The upstream oxygen sensor picks this up, and sees a lean condition. As the engine warms up, the leak somewhat seals. Until the next cold start

Of the 2 things I mentioned, in my experience, the intake gaskets are FAR more likely to cause rough idling

But an exhaust leak can most definitely cause fuel rims out of whack. Seen it plenty of times

Another thought . . . not every single vehicle will illuminate the check engine light for knock sensor code.

Exhaust manifolds have both been replaced and all bolts are good. (upstate NY salt capital of the universe, also this was at one point a plow truck for a lawn care company)
Yeah I won’t be taking up smoking any time soon… lol I’ll re-check the wiring (i’ve wiggled and jiggled it as well as any vacuum lines repeatedly over the years looking for a culprit)
I need to get an OBD2 cable and the software to data log as was suggested…