Newbie could use help with a 2003 GMC Sierra

[james38340]

Hello all I am a complete newbie to this and I am trying to learn from reading you guys posts. I have a stock 2003 GMC Sierra 4.3L V6 truck and my father passes away and left it to me. It has oversize tires on it so I did change that with the tire size editor and it made a big difference I think cause now it does not Rev so high before changing gears (Shifting). It seems to me to be weak on power and I was wondering if some of you more experienced guys could take a look at this tune file and adjust it so that it has a little more power. Please don't flog me for asking I do not mean anyone any harm. I am a totally newbie and all help and suggestions will be greatly appreciated.



2003 GMC Sierra Stock OEM File.hpt. Thanks James

[james38340]

One thing I might add is my father had the cat backs cut off and went back with just straight pipe so my MIL light stays on and that is all it had had done...other than that and titres is OEM. Thanks guys for your help! James

[camaroguy28]

what is the code for the mil light that is on?

[james38340]

what is the code for the mil light that is on?

I have a P0128 Temp below or something and the Catalytic converter codes should be present but when I changed my tire size it erased them for now but sure they will be back cause I do not have them no more. They got removed so no Cat Backs at all. Thanks James

[camaroguy28]

so to clarify, you have no catalytic converters on the truck, and no rear oxygen sensors? as far as the p0128 im not sure why that would come on. I would read more about that code and see what the common reason it triggers for that vehicle is. I would assume that the thermostat may be stuck open or something off that nature not letting the motor heat up all the way.

[james38340]

Yes that is correct no catalytic converters they are gone no rear sensors still have front. Yes I think you are also correct with the thermostat also possibly being stuck open too. I will look into that. Thank you for taking time to reply to my thread. Should I turn off my O2 Sensors in the DTC's? If so do I just select No Error reported and uncheck the box?

[camaroguy28]

You're welcome for the replies, I would do just as you say and turn them off in the dtcs. just select no error reported for any of the bank 1 sensor 2 and bank 2 sensor 2. Make sure that they are the sensor 2 because those are the rear ones. I never uncheck the box, but I don't think it hurts anything to do so.

As far as making the truck feel a bit more peppy, you can adjust the power enrichment tab in the fuel tab of the engine section. Set the hot and cold tables to something more like 50% because right now they are at 90%, then change the eq ratio to something closer to 1.15 for all rpms instead of what it is at now. right now it is commanding a much richer mix than it could be. next change the enrichment rate to something around 2.5 so it comes to the commanded ratio quicker, then change the delay rpm on the power enrichment to something like 500 rpms and the delay time to 0. that way you have no delay on the delivery. The next thing to do would be disable the cat overtemp settings. You do this by going to the temperature page on the fuel tab of the engine section and change the master switch to disable. see what that does for the truck and we will go from there. you could also play with the torque reduction settings too and that might help it feel a bit better.

how does the shifting in the truck feel?

[james38340]

You're welcome for the replies, I would do just as you say and turn them off in the dtcs. just select no error reported for any of the bank 1 sensor 2 and bank 2 sensor 2. Make sure that they are the sensor 2 because those are the rear ones. I never uncheck the box, but I don't think it hurts anything to do so.

As far as making the truck feel a bit more peppy, you can adjust the power enrichment tab in the fuel tab of the engine section. Set the hot and cold tables to something more like 50% because right now they are at 90%, then change the eq ratio to something closer to 1.15 for all rpms instead of what it is at now. right now it is commanding a much richer mix than it could be. next change the enrichment rate to something around 2.5 so it comes to the commanded ratio quicker, then change the delay rpm on the power enrichment to something like 500 rpms and the delay time to 0. that way you have no delay on the delivery. The next thing to do would be disable the cat overtemp settings. You do this by going to the temperature page on the fuel tab of the engine section and change the master switch to disable. see what that does for the truck and we will go from there. you could also play with the torque reduction settings too and that might help it feel a bit better.

how does the shifting in the truck feel?

I will do all of that to it soon sir and thank you a lot for the help in getting me stated. The truck did shift out kinda hard but the tire size was not changed and my father put larger diameter tires on it but I corrected that with the Tire wizard already. It shifts much better now not so high on RPM's like it was with the old tune with larger tires. I think it shifts smoothly but let me do these adjustments and see what we get. Thanks sir! James

[james38340]

I just made those changed to your exact recommendation and fixing to go test drive it! Thank sir will let you know soon how it feels. Thanks James

[james38340]

You're welcome for the replies, I would do just as you say and turn them off in the dtcs. just select no error reported for any of the bank 1 sensor 2 and bank 2 sensor 2. Make sure that they are the sensor 2 because those are the rear ones. I never uncheck the box, but I don't think it hurts anything to do so.

As far as making the truck feel a bit more peppy, you can adjust the power enrichment tab in the fuel tab of the engine section. Set the hot and cold tables to something more like 50% because right now they are at 90%, then change the eq ratio to something closer to 1.15 for all rpms instead of what it is at now. right now it is commanding a much richer mix than it could be. next change the enrichment rate to something around 2.5 so it comes to the commanded ratio quicker, then change the delay rpm on the power enrichment to something like 500 rpms and the delay time to 0. that way you have no delay on the delivery. The next thing to do would be disable the cat overtemp settings. You do this by going to the temperature page on the fuel tab of the engine section and change the master switch to disable. see what that does for the truck and we will go from there. you could also play with the torque reduction settings too and that might help it feel a bit better.

how does the shifting in the truck feel?

Sir I just drove it after the recommended changes and it is Better than before! It comes up to 60 Mph faster and is better response. The shifting seems to need some adjustment it shifts at about 3K to second gear and goes into 3rd gear much faster it seems only about 3.5k RPM kinda sluggish on that shift but overall the changes helped me out. I used the exact numbers you gave me being I really don't understand the tune mind of thought just yet but I will get it! Thank you again for your time and training me some as you have. I do have access to some tune files for the 5.3L V8 from a 2013 Z71 Silverado that has been tuned at a tune shop if you want a copy of it send me your email sir and I will share it with you for helping me. Thanks James

[camaroguy28]

Can you post the log from that run you did with the new tune? If not, can you run it again sometime then post that log so I can take a look at it?

You are welcome for the input, I hope I can keep helping you out.

[james38340]

Can you post the log from that run you did with the new tune? If not, can you run it again sometime then post that log so I can take a look at it?

You are welcome for the input, I hope I can keep helping you out.

Yes I sure will when you say log are you talking the scan or the tune file? I posted the updated tune in the previous post. Is it not showing up or do I not understand yet lol?

[james38340]

I think I understand now sir...I told you I am new 100% but after thinking on it I think you want me to run the truck using the scanner and save the data...Duh should known. I will do that tomorrow and post it sir. Sorry for not knowing but I'm learning. Thank you for your time. James

[camaroguy28]

No problem man, and that?s exactly what I meant. Just post up the scan and I will see what it looks like.

You said it was shifting around 3,000 rpms, were you at wife open throttle? It should rev to 5000 by looking at the tune file. The only time it commands a shift at 3000 is in 4 wheel low.

[james38340]

camaroguy sir I could not wait so I went on the road and done a couple WOT runs and it is better than I thought cause I just drove it before with kinda regular and then hitting the pedal. I rand WOT once up road then turned around and did it once more on the way back and recorded it all. Hope this is helpful for you sir! Thanks James and if not tell me what you need me to do,

[james38340]

I really needed bit more road. My mistake I was not WOT when I said it shifted early in my earlier post sorry my mistake

[5FDP]

Just a FYI if you are using 3.6 or newer it will auto save an log file, you just have to find it in your files.


Things to note from the log, you are lean per the fuel trims. Check for vacuum leaks or exhaust leaks before or after the front o2 sensors. Showing +14 LTFT and even +20 is saying that there is a vacuum/exhaust leak somewhere if everything on the vehicle is stock.

Add knock retard, injector pulse width avg and commanded AFR to your logs, even do cylinder airmass too.


The P0128 is for sure the thermostat, it's only running around 170-175 degrees. That's about 15-20 degrees below where the computer thinks it should always be running.

[james38340]

Thank you sir I will look into that more and I will add those to my log for the nest run. I am using the latest software v4.0 and I do not know exactly what you are telling me about it auto saving it but will try and figure it out LoL. Thanks sir! James

[james38340]

Just a FYI if you are using 3.6 or newer it will auto save an log file, you just have to find it in your files.


Things to note from the log, you are lean per the fuel trims. Check for vacuum leaks or exhaust leaks before or after the front o2 sensors. Showing +14 LTFT and even +20 is saying that there is a vacuum/exhaust leak somewhere if everything on the vehicle is stock.

Add knock retard, injector pulse width avg and commanded AFR to your logs, even do cylinder airmass too.


The P0128 is for sure the thermostat, it's only running around 170-175 degrees. That's about 15-20 degrees below where the computer thinks it should always be running.

Sir I added the logs for the review. I ran hard a couple times just FYI and then just normal driving because I was going around my block out in the country sir. Hope this is helpful to trained eyes! Thank you for your input! James

[james38340]

I just wanted to thank firstly Camaroguy28 for your help sir and your time in helping me. Secondly I want to thank 5FDP for your input as well sir. I just wanted to tune my fathers truck that he left me because it was so dang sluggish I thought. I by no means am I trying to make it a hot rod just a bit stronger was my intentions. With what you guys have helped me with thus far is great. I can defiantly tell a difference in the ole girl. I will take the advice and look for some vacuum leaks or exhaust leaks and change the thermostat to an OEM 195 and try some more adjustments if you all have any other recommendations. Like I said the scan file may be long cause I drove her around the block basically and WOT it a couple times in the process. Thanks Guy's! James