Fuel delivery issue........... I think?

This may sound a little hair brained, but bare with me, I am still doing some trial and error to try and figure it out. I have noticed lately that when I get down to around a quarter tank of gas it seems like my truck runs like sh*t. So I have really paid attention to it in the past few days. I ran it down to right under a quarter tank and did some WOT and just really paid attention to the throttle response and everything. Then I put gas in it and brought back up to around 3/4 of a tank and it runs totally different, better it seems. Junk in the gas tank? I just put new injectors in it maybe 5000 miles ago, so I wouldn't think those are the problem. It has right at 140,000 miles and I haven't done anything with the gas tank or fuel delivery system. I keep 93 octane in it because of the tune and occasionally run injector cleaner through it, but haven't since I put the new injectors in. I have been noticing some torque converter chatter at light throttle (see this thread, http://www.ls1truck.com/forums/mecha...-question.html) and noticed that it goes away when I put gas in it, any connection there, or am I just delusional? Any ideas? Any comments would be appreciated.

'05 CCSB Z71, 5.3, tuned, LT's, 3" exhaust, built 4L65E, TBSS intake, LS2 injectors, 90mm corvette TB (P&P'd), 150 wet kit with plate, MSD 8.5 wires, NGK TR6's, about 2 degrees timing pulled for the spray

In the tune is a section for high octane and another for low octane. When you have low fuel and shut the truck off and refill the tank the PCM defaults back to the high octane tune section. If it encounters knock or ping then it drops down to the low octane tune. I'm not exactly sure of how the logic works to get it back into the high octane section of the tune again. I'm not sure if that is what your issue may be.

It's strange that the engine would experience knock if his tune has a couple degrees pulled out for N2O, especially if he is running 93 octane fuel. I would probably start by checking fuel pressure at a 1/4 tank, and then check it with a full tank. See if there is a diff. there.

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Originally Posted by RedHeartbeat

In the tune is a section for high octane and another for low octane. When you have low fuel and shut the truck off and refill the tank the PCM defaults back to the high octane tune section. If it encounters knock or ping then it drops down to the low octane tune. I'm not exactly sure of how the logic works to get it back into the high octane section of the tune again. I'm not sure if that is what your issue may be.

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I was unaware that the computer would do that if it sensed knock/ping. I had someone do the tuning for me so I will contact him and see what he says.

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Originally Posted by truckinL33

It's strange that the engine would experience knock if his tune has a couple degrees pulled out for N2O, especially if he is running 93 octane fuel. I would probably start by checking fuel pressure at a 1/4 tank, and then check it with a full tank. See if there is a diff. there.

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Way ahead of you, I stopped and bought a fuel pressure kit this morning and going to check it tomorrow while I am off. What is optimal pressure for a stock fuel pump? Playing devil's advocate, what do you think would be the problem(s) for findings in either direction, good pressure all the way around, or, good pressure at full, low pressure at 1/4?

I don't know what the fuel pressure is supposed to be at, I would just be checking to see if there was a diff. at the diff. levels of fuel. If there is, then I would take it to the tuner and see if he can figure something out with the laptop connected to the truck on a cruise or something. For me it's easy because, I can make a quick call to our GMC dealer we have and they can give me any kind of fuel press. info and such.