Missing after WOT???

[1BadBowTie]

Ok, i am at a loss, I have a 99 1500 with the 5.7, with 119000 miles, and when u floor it to 70/85 mph, after i let off it starts missing pretty bad, then after getting back on it slightly it fixes it self. i changed plugs, wires, cap, rotor, threw in some injector cleaner, and i cleaned out the EGR Valve and its still doing it??? any known reasons for this???? also not sure if this is related, but i had a buddy follow me one day when it did it,and he said "it smelled like gun powder" ???? any help would be great!

[webbch]

This is just a WAG, but at WOT, the engine computer goes into open loop mode IIRC, ignoring the O2 sensor inputs. I'm not sure how long after you stop WOT that it returns to closed loop mode and incorporates the O2 sensor readings, but my WAG is that the O2 sensor response may be a little bit sluggish, although that alone wouldn't seem to explain why it doesn't eventually settle back down until you get on the throttle again. see here for how to test.

Could a really rich air/fuel ratio cause the type of "missing" that you're seeing? Can you replicate this behavior while in park and revving the engine?

Any engine codes getting thrown?

[1BadBowTie]

well, that in a round about way sounds like my probme, but i know when i sit at idle, its kinda hesitant, not really a miss, when u rev it its fine, i only notice it after its floor and its kicked down into torque-lock. but one time i did it bad enough to kick on the SES light, and the code was "cyl 4 misfire" i erased the code and it hasnt come back on since. so im at a lost of what this could be, im also experiancing piss poor mileage too, like 12 on a good day, recently its been 10 / 9 ish. so it does sound like it could be running rich

[corning_d3]

Had a tune-up recently?

[webbch]

The piss poor mileage *might* have something to do with an O2 sensor - might be worth checking out. The engine computer will use this data in conjunction with the MAP sensor to determine the appropriate air/fuel ratio to send to the injectors. Once your idle problem is fixed, if mileage remains poor, I would look into testing these two sensors.

As far as the idle is concerned (and after thinking about it, this could be causing the initial symptom you were describing as well), I'd be looking toward the Idle Air Control (IAC) valve, as the symptoms you describe are sometimes the result of a dirty IAC valve and/or vacuum leaks.

Finally, might be good to just verify proper fuel pressure at the injectors. I have an older rig w/TBI, so not sure on the spec for you.