Sluggish 2003 1500hd

tahoesub1 · July 13, 2009

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.

bill190 · July 13, 2009

So far as the exhaust, I suppose you could remove a forward oxygen sensor (leaving it electrically connected) and perhaps use a coat hanger or wire to hold it over the opening it was in (so the exhaust coming out of that hole would hit it and maybe it would keep monitoring the exhaust?)

Then try driving and see if the additional opening in the exhaust gives you more power. The thing is if the oxygen sensor is not reading the exhaust inside the pipe, would this in itself change the responsiveness?

I suppose you could test that by leaving the oxygen sensor installed and disconnecting it electrically. Then drive test.

Then remove it from the hole and drive test again. (again not electrically connected.)

This will give you a check engine light though.

Then so far as the knock sensor, the way I have read these work is if they detect a knock, the engine computer will retard the timing, then slowly advance it as far as it can without it knocking again.

Sort of like a little kid who will push it as far as he can until you explode, then learn how far he can push it!

And these would look for common noise and distinguish a knock from that constant common noise based on a frequency, nothing to do with the resistance of the circuit. Sort of like speakers might have a slightly different resistance, yet the sound comes out ok. And a person listening to the sound would be able to distinguish what sounds they were even though the speakers had a slightly different resistance. It is the "vibrating" of the speakers which counts!

Anyway if the knock sensors are in fact sensing knocks and retarding the timing, this could be shown with a GM dealer type scan tool.

Then the following engine computer sensors are of interest...

(I don't know which specific sensors you have)

-The Intake air temperature sensor (IAT) retards timing when the air is hot.

-The atmospheric (barometric) pressure sensor adjusts for lean/rich

-The coolant temperature sensor adjusts for lean/rich.

-The forward oxygen sensor(s) adjusts for lean/rich.

-The manifold absolute pressure sensor requests more fuel with a vacuum (load).

-The throttle position sensor requests more fuel as the throttle is depressed.

So if one or more of the above sensors are not working as they should, I suppose that could put a damper on things.

Also might want to check the electrical resistance on the injectors and compare. Here are more fuel system checks...

http://www.asashop.org/autoinc/june98/techtotech.htm

tahoesub1 · August 13, 2009

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 feul 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!

uniteam · August 22, 2009

I have a similar problem with my '02 Sierra 1500 HD 6.0L .. I feel its performance like a Turtle I did the complete tune up too ... and it was running 4.10 Gears with 37's and I swap to 4.88 thinking it was the problem.. but It runs more free just a couple of tests, but then it starts again sluggish off the line ...