03 5.3 bucking/backfiring

[Boss302]

hello guys- ive got a problem- i purchased a 68 shevy p/u with a 03 5.3 motor and trans from a tahoe- it is installed/runs drives/ problem is it bucks/backfires under load.. if i disconnect the o2 sensors it will run good??it pulls a code for the o2 sensors about them not going into heat cycle? im new to this engine and would appreciate any help. the wire harness is from squire performance< never heard of em. the computer is out of the same tahoe and was sent to some place in houston for a reflash/unused stuff removed. this is all i really know about it. thanks Lee

[RedHeartbeat]

Start with the fuel pressure and make sure its correct, 50 psi at idle and 56 at low vacuum under acceleration.

[Boss302]

i think it was at 40psi at idle and 45 w/ vac removed. would that cause the problem?

[RedHeartbeat]

Very much so. Ask DamnYankee about low fuel pressure, you lose big time performance because of it. You may have a ruptured fuel line in the tank if you have an in-tank fuel pump.

[Boss302]

what is agood fuel pump to run? in tank or external? would a aeromotive a1000 be too much? thanks for the help . Lee

the pump used is a 87 model pump and tank otta a suburban.

[RedHeartbeat]

You have got to get an inline Walbro or the Aeromotive if you want to spend that much money. Be sure you have a return line for the excess fuel pressure. I guess I feel obligated to ask this question, so here goes... are you running fuel injection or does this engine have a carburator?

[Boss302]

<div class='quotetop'>QUOTE(RedHeartbeat &#064; Mar 15 2007, 08&#58;12 PM) [snapback]86385[/snapback]</div>

You have got to get an inline Walbro or the Aeromotive if you want to spend that much money. Be sure you have a return line for the excess fuel pressure. I guess I feel obligated to ask this question, so here goes... are you running fuel injection or does this engine have a carburator?
[/b]

this engine is running full efi electronics from the 03 tahoe.
its runnig a return line to the tank. the pump in the tank/and the tank itself are from a 87 subarban.i may try running an inline walbro 255 like in my 68 mustang. do you guys sump your gas tank? or run some type of pickup tube. thanks again.

[RedHeartbeat]

I&#39;m running a stock 93 fuel tank that starves for fuel if you have 1/4 tank and mash the throttle. That is the reason GM went to the bucket style fuel pumps, that and it keeps the pump cooler while submersed in the bucket full of gasoline.

[farmtruc]

If your pcm is setting ses codes, make sure that doesn&#39;t have anything to do with it. i&#39;ve seen trucks run ok with 45psi, no bucking or surging, so that shouldn&#39;t be a big issue, it&#39;s not good, but it shouldn&#39;t buck with 45psi.
Since your truck had a swap, i bet the o2&#39;s were changed or are going bad, or were sitting around a while. When the pcm senses a rich or lean condition it will over compensate the other way.

If you have access to some kind of logging, check to make sure your O2&#39;s aren&#39;t stuck at zero and are not floating around 400mv&#39;s. A good O2 will toggle from +-50 to 900+ mv&#39;s, up and down like a nice sinewave.

Your fuel pressure doesn&#39;t look good either, if you currently have an intake pump, get yourself a walbro 255 pump, it&#39;ll work.

Good luck,
allen

[Boss302]

its throwing a ses light for the o2&#39;s. nothing for the fuel pressure though. i did find that the fuel is running through a box of some sort for fuel economy?? but its not plugged in. i may try bybassing it and see what fuel pressure is then. thanks for the help guys. Lee