I have a 2003 1500HD with a 6.0, 3.73, 140K miles and all stock. Truck is new to me, and feels sluggish compared to my Z71's, one with a 4.8 and the other with a 5.3. Sluggish off the line, holds gears for a long time at light throttle, otherwise seems to run good, no noticeable knocking or pinging. No codes at all, replaced plugs, wires (all AC Delco), air filter, fuel filter, pcv valve, all fluids. MAF looks clean, throttle body is nice and clean, brake drag seems normal. Fuel pressure is within specs at idle and with engine off, exhaust backpressure at 2000 rpm is about 0.5 to 0.7 psi, which is slightly above spec. Checked ohm reading on knock sensors, had one at 100k and other at 111k. Spec is 93k-107k. Would one knock sensor slightly out of spec cause these symptoms, or could the exhaust backpressure being slightly elevated cause this? Hate to dig in to these if they are not the cause. Any help is appreciated.

They are just like that. Mine is still sluggish on the odd day. Take your intake tube off and check for black gunk inside your intake manifold and around the throttle body. If there is, clean them up as best as you can. Also put some injector cleaner into the tank and run it hard up some hills. These are not fast engines stock, and your truck weighs 1000 lbs more than a half ton.

I have done a few little things to mine, and it is pretty peppy now. I also find that it really responds to how I drive, when its been in the city a while it shifts at lower RPM and is more sluggish. After working it at the farm, the pedal almost pulls away from my foot. The way the 4L80e shifts is a bit goofy on these trucks, that is the biggest thing, sit in any truck form the 90's with the same tranny and you will see what I mean.

Use some Sea Foam to clean up the intake and crap. Do a search on it here.

Im guessing you dont have access to EFI Live do you? If so you can log it to see if its going into KR all time. That would pull timing. Put a lower T Stat in it and that will help with KR also.

If I had to guess its fine and nothing wrong. Clean the MAF with MAF cleaner and swiss cheese your air box if your not running a CIA.

Also are your plugs gapped correctly? Its not missing right?

Use some Sea Foam to clean up the intake and crap. Do a search on it here.

Im guessing you dont have access to EFI Live do you? If so you can log it to see if its going into KR all time. That would pull timing. Put a lower T Stat in it and that will help with KR also.

If I had to guess its fine and nothing wrong. Clean the MAF with MAF cleaner and swiss cheese your air box if your not running a CIA.

Also are your plugs gapped correctly? Its not missing right?
I had a 2000 3/4 ton with 6.0 and I put a diablosport programmer flowmaster dual e xhaust and wasn't very sluggish at all.

my 2000 1/2 with a 5.3 9inch lift and 35's would beat my 04 2500hd with the 6.0. The 1/2 was a 4x4 ext cab sb with 4:10, the 2500hd was a 4x4 cc/lb with 4:10's also.

These trucks are dogs compared to their 1/2 ton counterparts. One reason is the trans, the gears are taller, plus there is so much torque control management on these things. You need a custom tune to wake it up. Stay away from Hyperjunk or any other canned tune, go to Westers or any other recommended tuners on this board. Dollar for dollar you can't beat a good tune.

Yep, the 4L80E trans has a tall first gear plus eats a ton of hp compared to the 4L60E with the 5.3 and the 4.8 also with the HD an extra 1,000lbs makes for a slow truck. EFI Live tuning can drop 0-60 a full sec. If you want the tuck to move put in a cam, headers, and tune. Makes a huge diff, more lowend power and way more mid and top. Putting in a cam is easy with the LS engines.

My buds cam, headers and tuned 6.0 HD http://www.youtube.com/watch?v=eJnY0fPcAZ8&feature=related

This truck will run with a 125+hp tuned D-Max from a 20 mph roll on up and pulls harder then the Duramax after 85 mph.

Thanks for all of your tips. I am definitely going the custom tune route and am waiting for Blackbear to free up a scan cable to start the process.

You need a custom tune to wake it up. Stay away from Hyperjunk or any other canned tune, go to Westers or any other recommended tuners on this board. Dollar for dollar you can't beat a good tune.

Definately good advice and by far the best bang for your buck

Just thought I would close out the thread. Hooked up with Blackbear and did the scan cable tune. Justim immediately spotted a lean condition and directed me to check fuel pressure and the MAF sensor. Turns out the MAF was not working properly, but still well enough to fly under the PCM's radar. New MAF made the truck much more driveable immediately. The next scan showed the improvements by the MAF but yielded a intake manifold leak. Will replace gaskets later as it not real bad now. Justin then tuned a PCM for me. The results are great, more power all around and mileage does appear to have improved. Thanks to Justin @ Blackbear for a job well done!