random misfire p300 - I have a 2001 silverado 5.3, in 2wd. Recently, I had some issues with bank 2, and I swapped the upstream O2 on that side, and ...

I have a 2001 silverado 5.3, in 2wd. Recently, I had some issues with bank 2, and I swapped the upstream O2 on that side, and it cured it. THis was a month ago. NOw, I was getting random misfires p0300 and a heater wire bank 2 O2. A little history on the truck:

Mods: Pacesetter long tube headers, ORY (delete downstream o2's and cats), Powerbond 25% balancer, MSD 8.8 wires, TR55 plugs, Custom Tune by RPMonline through my predator tool, K&N filter with stock air box.

What I did after I got the misfire/ O2 code: Cleaned the MAF sensor, changed plugs, checked plug wires, checked headers for tightness and leaks, checked exhaust connections for leaks (none apparent), changed oil and filter, cleaned and reoiled(lightly) K&N air filter.

After all that, nothing changed. I watched as I got misfires on all cyclinders. Also, the fuel trims, both short and long term, were very high on bank 2. Bases on that, I decided to get another O2 sensor for bank 2 and swap it this afternoon. I did, and the fuel trims on bank 2 went back to normal. Problem is, I'm getting misfires still (no code) on cylinders 1 and 7 ONLY. I watched the live data for 5 minutes, and those were the only misfire cylinders. So, I swapped the #1 coil pack with the #3 coil pack, and fired it back up. Misfire remained in cylinder 1. So, it's not the coils. Not sure what to do from here. I have a new fuel filter I haven't installed yet, but i'm thinking that isn't it as it would effect all cyclinders, not just 2. What i'm thinking now:

Injectors????
Camshaft Position Sensor??
Crankshaft Position Sensor???
Two of the colder spark plugs didn't clean them selves up?

All the misfires are on Bank 1 now, so it's not an O2 sensor issue anymore. What do I do?????????????????????//

Just a quick update, but I pulled the #1 plug, and it looked fine. I decided to head to the store and pick up a couple icecream sandwhiches, and 2 miles down the road, misfires on all cylinders came back. (had the predator hooked up and monitoring the cylinders. PLEASE HELP!!!!!!!

Have you checked for vacuum leaks? The fact that it is all the cylinders leads me to think it is not (but it could be) an actual ignition problem. While you are at it, check the fuel pressure to make sure it is within specs.

I hooked a pressure gauge up at idle, and it was right around 49 to 50 PSI. It's a cheap Harbor Frieght pressure gauge, so take it for what it's worth. I've got a fuel filter that i'm going to put on it this afternoon for good measure.

Something i'm going to try this afternoon is hook the pressure gauge back up, recheck the pressure and monitor the fuel trims for a minute or two. (i'll log the data) Then, remove the fuel pressure regulator hose, and see what happens. The PSI should jump at that point. If it doesnt jump, possible intake leak?

Also, when you say check the vacuum lines for leaks. This may sound like a stupid question, but, it's a tad difficult to "see" a vacuum leak in a hose. what is your preferred way to test for vacuum line leaks?

So, I hooked up the pressure tester, and at an idle it was reading 50 PSI, and the short term trims on both side were even, and right around 0.00 to -1.5. I thought that to be normal. Pulled the vacuum line, and the pressure jumped to 59 PSI, and the short term trims didn't move much, just to the positive side of 0.00 to around +1.50

Seem pretty normal eh? I was going to put the fuel filter on this afternoon.

OK, changed the fuel filter for good measure, and no change. Still getting misfires on all cylinders, but, it seems that most are coming from cyclinder 1. Also, another P0300 code. Looking for ideas now guys.

I restored the factory PCM programming, and loaded the canned 87 octane tune. Drove it around for a bit. What I noticed is, the throttle response and power was noticeably better. (imagine that) what did not go away, however, was the misfires. The tune might suck, but that doesn't seem to be causing the misfires. I also noticed that, and reconfirmed, that most of the misfires are happening at idle, or at light throttle. During excelleration, i'm not seeing any misfires on any cylinders.

I am very surprised that, all the knowledge and high speed/ low drag people on this site, and no one has any input. I've done all I know to do, this old Vet needs some help.

Its hard to diag something that complicated not being there to watch things and try things. Have someone to spray carb cleanerd around where the intake bolts to the heads. Those engines where bad about the gaskets to go out, so bad in fact gm redesigned them some how. And ive seen the intake warp on them also while its cold you might not see misfires on all cylinders but once you drive it a few miles and get it hot the plastic can distort just enough to give you vaccum leaks on the other cylinders and therefore cause random misfires. The carb cleaner should burn like gas if you have a vaccum leak and the scan tool stop showing a misfire and if you have a good nose you might even be able to smell it in the exaust.

I left the canned 87 octane tune in, and the truck drives great. I'm still showing misfires on the predator, but i'm not feeling them during driving. Filled up with some fresh fuel. What gives?