Ok guys I have a 99 Z71 with the 5.3, it has 126,000 miles and I'm having a problem. It has set off random misfire codes in the past, and it would run rough but they would go away after a couple of days. Well in the last 2000 miles I replaced my plugs with delco stock plugs gapped at .060. It ran great for 1500 miles or so and all of a sudden I have a P308. I drove it for a while expecting it to clear up. Then the fuel pump went out yesterday(low fuel pressure and the truck would not get out of its own way) so I figured that was the problem. I replaced the fuel pump and it runs better but still has a rough idle and miss. I just changed the plug wires and the #8 plug in case it had cracked or something. And it still has the miss. The truck is stock exept for a K&N drop in replacement. Does this sound like a bad injector or a bad coil? I have also replaced the PCV valve. Here is the symtoms. It idles ok, but you can feel a miss, it has lost a little low end punch, and it will pop at the top of 1st gear at WOT or backfire a little at a very low RPM when cruising. (like 3rd gear at 25 or so, when the motor is straining to push the truck right before the point where it will downshift) The fuel filter is also new, and the codes have been reset. Today I also had the codes read again and this time it was just P300. And the SES light does flash on occasion. Any help would be great.
J.D.

 

Hey J.D.

Do you have autotap or something like that? If so, look for which cylinder has the misfires. If it is isolated on that one cylinder, swap the coil packs to a different cylinder. See if the misfire moves with the coil. You could move injectors as well. Have you changed your fuel filter?

 

while its running pull one plug wire off at a time to figure out which cyl. is giving you the miss. Then do a compression test on the dead hole that you find.and or swap coils