BlackZLS1

iTrader: (5)

I've searched around and havnt found anything quite like my situation.

First off, its a 99 sierra with the 5.3. I just bought it with 160k miles already on it. Symptoms #1: right after start-up, if I give it more than just a little bit of throttle, it craps out just as if its running out of fuel. If I drive easy for about another 30 seconds to a minute, it gets better and I can go ahead and drive normal. It will do this as long as it has set for at least 20 minutes or so. An immediate stop and start doesn't seem to do it.

Symptom #2: once up and running normally, it has great power up to about half throttle, at which point any further it seems like again its running out of fuel, but not as obvious as symptom number 1. What this does is just make it feel like a turd, when I know it shouldn't be based on how good it runs otherwise at part throttle. Also, I hooked up my predator for my camaro to the truck, and while driving it I didn't notice anything unusual except the 02 sensor reading. The 02s look normal under the first 50% throttle, but then after that, start dropping drastically in mv's. Then at wot, all the 02 sensors go to 0mv. I know that cannot be right, because in my camaro, they stay around 880mv to 900mv at wot. Could this no voltage reading be the 02 sensors or somethin else? What gets me is all 4 sensors will go to 0mv, instead of just one if I had a faulty sensor. Or else all 4 are bad?

Also noteworthy, I check fuel pressure at the fuel rail. It sits at about 48psi after having driven it and it being warmed up. When I blip the throttle, it goes up to 60, then back down to 48 or so. Even revving it way up wot, it never gets any lower than that. Also once I got it, I changed the fuel pressure regulator and fuel filter because it was doing the notorious having to cycle the key before it would fire, the fuel pressure was bleeding off.

What could this problem be? Need opinions before I start buyin stuff.

Truckshop

iTrader: (17)

clean your mass air sensor or swap one from another vehicle

BlackZLS1

iTrader: (5)

Cleaned the mass air, didn't seem to do anything. The mass air is picking up upwards of 18lbs at wit and back down to around .8lbs or. 9lbs at idle and smooth in between. So not ruling it out, but not sure its it either. Still running dog lean at wot with new 02 sensors. Also noticed with the Marco scanner that the timing dips to below -14° for a split second during a quick blip of the throttle. Which means 14° after tdc, then jumps back up to 20-40° before tdc. My camaro never does this as it never gets below aorund 10 or 12 btdc. When the truck goes this much retarded, it has that sound that it has when it takes off before warm up. Next thing im going to do is let it cool down and then take off while watching the timing. Question is: what could cause the computer to back off timing so drastically? And it's only momentary once up to running temp.

BlackZLS1

iTrader: (5)

Wot* Matco*

1FastBrick

iTrader: (17)

Phantom Knocking would cause it to pull timing. Have you cleaned the throttle body? Usually takes care of the problem down low when you blip the throttle and it dies. Did you look at all the sensor values at idle and again under partial load??? Does the engine check out mechanically???

1FastBrick

iTrader: (17)

fuel pressure sounds about right

BlackZLS1

iTrader: (5)

I have not cleaned the trottle body, I can do that, just hadn't come to mind since everything else seems to be perfectly in order under the hood (mass air and filter seemed to be perfect). I did check and compare all the other sensors using my camaro ls1 as a reference. Everything else seemed about the same minus the 02 at 75%+ throttle, at which point the 02 readings drop to. 0-60mvs, instead of 880mv or so on the ls1. I looked at idle too, which all looks the same besides the timing on a throtle blip. The engine itself seems to be in great shape. The oil looks good and it is quiet and smooth, and actually fairly clean for 161k miles.

1FastBrick

iTrader: (17)

Front or rear o2 sensors go to 0

BlackZLS1

iTrader: (5)

All 4. The other day I reran the exhaust and deleted the cats and the rear 02s. At that point I out in new front 02s. So now its just the fronts, but the way the truck acts hasn't changed.

BlackZLS1

iTrader: (5)

So its just the fronts reading 0mv since that's all that's on there