Another '08 5.3 LC9 idle stumble

[Dzlhead]

Hello. I have done extensive web research and many forum searches here on this great site on my 5.3 stumble. I mainly just brose and drool over in the 60 - 72 forums. I'm at wits end and quite frankly the wife is getting annoyed at my obsession over this idle stumble that it has developed. I don't really like to drive it any more. Sorry for the long post, but I'm trying to cover very thoroughly on what I have done. Thanks in advance for any comments.

For the facts. Back in Nov of '14 I bought a used '08 Suburban LTZ with 99,500 miles on a LC9 5.3. We showed up twice unannounced to view it. I got to hear cold starts and very little ticking for maybe 10 seconds. No piston slap, yay. I checked the oil and full both times as it should because according to the sticker it only had roughly 100 miles on the oil change. It ran smooth and quite.

The first weekend I had it I installed a new air filer and I did ACDelco plugs (41-110) and wires (748RR). I figured a 100K mile tune up and I'm done. All so I knew that I did these important things no matter what was said to be done in the past. I used very little silicone on just the plug porcelain so the boot slid on easy. Before I put the plugs in I checked gap and I checked each fit (click ) of when the boot and plug are seated so I know that when on the engine I had the boot fully connected/seated. I silicone greased the coil boot as well and even made sure to release the air from the coil boot so that it is fully seated in the coil pack and so it won't work itself back out.

It ran good for about 1000 miles but since about 101K or so it has developed a stumble at idle in and out of gear. It now has 106K on it. So for about 5K I've been BOTHERED by this. I've used a spray bottle with water in the dark and NO stray arcs under the hood. I did use the SeaFoam spay into the throttle body. Noting to not move the throttle plate. I used Mass Air Flow spay on the MAF. I have ran soooo much SeaFoam (proper amount per tank/gals) thru the gas tank. I get gas at a very busy gas station so it's fresh gas. I've tried 87, 89, 93 octane. I've used Marvel Mystery in the oil during one 3,000 mile oil change and SeaFoam during the current oil change in the oil. Still no oil consumption at all, yay. I check the oil at every fill up and it's full each time. Oil pressure at cold/start up idle near 60, hot driving at 50 and idle 40 right on the head. There is very little lifter tick at start up. Even after a full 3 day weekend of not driving it. Just about 10 seconds.

I keep bringing up the oil because I've seen thousands of posts all over the web with a collapsed AFM lifter causing a stumble or rough idle. With my solid oil pressure, and very little lifter tick, I don't think it's a collapsed AFM lifter causing a rough idle.........or is it?

The fuel injectors all click just about the same. Could one of these be going?

Did I buy a bad set of plugs and wires? I've had the plugs out twice and no cracks in porcelain.

I have NOT done anything with the idle air control..........

A life long, trusted friend, that has been a mechanic for 20yrs now (not at any of the major car brand dealership) told me that a coil pack could be going, just not bad enough yet to throw a code. The truck pulls hard and smooth under acceleration. I baby it, I have never gone over 4000 RPM. If I pull a wire off a coil, obviously it runs bad, too bad for me to tell it that coil is the culprit. Do I buy one coil and switch each out til I find the miss fire? It feel like it's multiple miss fires, but NO codes, ever.

I took it to a GMC truck dealer 'cuz I figure they see this more than a regular Chevy dealer. Maybe Im wrong taking it to this dealer 'cuz I see that some of these year GMC's didn't have AFM. The dealer said it's fine, and they don't think it's the lifters, but the thing shakes so bad. I had the service guy and the tech both put their hand on the engine with me and the tech agreed it ran rough, but said without codes they can't do anything. HUH? WTF! I feel like they don't want to help me since I didn't buy it there. It's soooo annoying! I've wanted a Burb of this body style for soooooo long!............ I want my '97 Tahoe with 229K back...it ran better than this.

Thanks for letting me vent.
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