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Hi guys. Ok, I actually have an avalanche as well as a couple of silverados.
I have a 2003 5.3 avalanche that one day as i hit a set of traintracks a litle hard, the engine developed a really bad misfire. When idling, it will rattle the filings out of your teeth, but as soon as it gets above an idle while moving, it runs great. the check engine light comes on, but not while its idling. It usually blinks at odd times like when im off the gas and just coasting down.
The only engine code it throws is P300. I paid to take it to the local electronic specialist for diagnostics. All he could tell me was that the misfires were on 2,3,6 and 7. He said that it appeared that all the electronics were working correctly. He also told me that he said the truck was running like when a ford 5.4 skips timing. He said that the engine was firing at idle at full retard and that it was shaking soo badly because the electronics couldnt keep up with the timing. It ran ok when it was moving because the electronics had more room to play with as far as the timing goes. I kinda believe this because, it might idle like crap, but after it moves, it runs like a rocket ship and is as fast as my silverado ss.
i sprayed the entire exterior of the engine with starting fluid looking for an air leak and found nothing. He told me that it doesnt appear to be a coil issue either.

So, how on earth would this motor skip time? Obviously it didnt slip the timing chain because it would have smashed all the valves by now and wouldnt run at all.

I am open any suggestions
 
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