PO300 random missfire only under heavy throttle

[04SS99Denali]

was driving the old rig going up a steep mountain up state yesterday. everything was going good till the truck downshifted into second and the rpms hit about 4k. immediately it fell on its face and a few seconds later started shuttering with flashing engine light. i knew it was misfiring by the symptoms so i let off and it up shift. everything slowly dissipated after the rpms went back down. once i got over the hill i drove another 50 miles home just keeping the throttle under 3k and it was fine. no shutters / missfiring/ or anything.

pulled the code this morning and it was p0300. my question is what could it be?? if it was a lose plug wire / bad plug / bad wire it wouldnt have went away. could it be the distributor? i replaced cap / rotor about 1000 miles ago as well as plugs and wires. im all out of ideas i tested it out all morning and it wont come back.

truck is a 99 yukon 131k miles

 

[04SS99Denali]

lol no suggestions??

 

[willxfs]

No good advice to offer but my rig (1998 2 door with 260K miles) did something similar. I got on it hard one morning on the way to work. Rpm's probably hit 4500+ in second gear. When I pulled in the parking lot at work just a short bit down the road, the engine almost died it was missing so bad. Check engine light was on. I parked it and drove home after work. All was good. Never did figure that one out. Since then, tranny went out, pulled it and figured it was a good time to pull the engine too. Rebuilt it to 383 stroker. Still workin on the tune but its running again.

 

[04SS99Denali]

yea im gonna keep driving it till something happens i guess. i only use it like once a week on a leisure basis. i do plan on throwing a crate in it eventually once this ol 350 dies. might try to LS swap it

 

[SunlitComet]

your engine misfire history in the pcm will tell you what cylinder is acting up. use that to narrow which parts you need to test.

 

[04SS99Denali]

yea i havnt dug to deep with this issue since it only happened 1 time. i put probably 400 miles on the truck since then it hasnt acted up again. very weird but ill leave it as it is. i checked all plugs/ wires again none are lose or anything. if it happens again i will have it hooked up to a lap top to really see whats going in that ol 350.

 

[Tonyrodz]

was driving the old rig going up a steep mountain up state yesterday. everything was going good till the truck downshifted into second and the rpms hit about 4k. immediately it fell on its face and a few seconds later started shuttering with flashing engine light. i knew it was misfiring by the symptoms so i let off and it up shift. everything slowly dissipated after the rpms went back down. once i got over the hill i drove another 50 miles home just keeping the throttle under 3k and it was fine. no shutters / missfiring/ or anything.

pulled the code this morning and it was p0300. my question is what could it be?? if it was a lose plug wire / bad plug / bad wire it wouldnt have went away. could it be the distributor? i replaced cap / rotor about 1000 miles ago as well as plugs and wires. im all out of ideas i tested it out all morning and it wont come back.

truck is a 99 yukon 131k miles

I have an 01 express van and it has the same motor as you, but mine is a 5.0. I kept having a similar issue and mine turned out to be the distributor. The lower gear was worn and it had some play in the shaft. Since then, no more misfire. Try checking yours. Keep us updated.

 

[04SS99Denali]

I have an 01 express van and it has the same motor as you, but mine is a 5.0. I kept having a similar issue and mine turned out to be the distributor. The lower gear was worn and it had some play in the shaft. Since then, no more misfire. Try checking yours. Keep us updated.

thats what i was thinking. i just pulled my distributor out when doing my intake gaskets like 2 - 3 months ago. it didnt look to bad so i put it back in i replaced the cap / rotor. if i put it in wrong that truck would be mis firing / running like shit all the time not randomly. so i ruled out my own mess up lol.