ZRated

This is a new problem in my quest to solve the random slight "stumbling" of engine when in Drive (sitting) or sometimes in Park/Neutral.

Read quite a few forums on this and decided to do my own thing first. I decided to pull the plugs and take a look at them since I don't feel purchasing a bunch of diagnostic equipment when I don't even have a code. My old OBD2 stuff isn't reliable anymore for the newer vehicles (did a LOT of work on LT1 cars back then).

I saw that the #1 and #4 plugs were caked in white gunk. The rest of the plugs were fine and normal wear. I showed the nasty plugs to my mechanic and he said "quit using that crap cheap gas". I had been running the "up to 10% ethanol" stuff for a little while when it got real cheap and that is when I started getting the "stumble". I usually run 92 octane 100% since my Tahoe is not a FLEX Fuel.

So, I changed the plugs (by far the easiest plug swap I have done except for #8). Right after I noticed a strong raw fuel smell coming from my Tahoe after it was shut off. It would fill the garage after sitting. I had torqued the plugs at 16ft-lbs (manuals say between 11-15ft lbs.). I usually just do plugs by hand but that was on the cast iron heads I was used to. I also did the plug change on a cold engine and used a little dab of oil on my finger for the threads. I started reading that a lot of people got this fuel smell after a plug change and the issue was the plugs were either too tight or not tight enough and vapor was seeping out through the threads. Most re-torq'd and say it solved the problem. Personally I have never heard of this without some major indication that something was wrong on the head. So I rechecked the plugs. I have about 2,000 miles since the plug swap and they were clean as a whistle. I decided to use anti-seize this time and put them back in @ 16ft lbs. No cross-thread etc. They came out and went in by hand to the point of being seated. I fired up the Tahoe and within 3-4mins I could smell that strong raw smell again.

I have been all over the engine bay and underneath looking for any leaks or indication of fuel. Nothing. I have been told that an O-ring on the injector could be bad etc. etc. Either way something is dumping or causing this raw fuel smell. I have no codes and engine seems to run fine except that the stumbling is still there randomly.... I have also run 3 full tanks of 92 100% with injector cleaner after the plug swap.

Any ideas where to look next? I have a strange feeling that when I find the cause of the stumbling problem it will also be my fuel smell problem. I am thinking either ignition or injectors. All plugs are getting spark and show slight wear evenly. None had any residue or a heavier odor than another.

Is there a way to test my coil packs without spending an entire day? How about injectors? I have a 1 in 8 chance here of replacing one and it being the right one..and I won't waste my time with those odds.

This Tahoe has about 180k miles on it and I have had since 30k. I take very good care of my vehicles and they tend to last a long time from this. I had an Expedition with 220k on it but seemed to have a coil pack go bad every year until all were replaced eventually. When a pack was going bad on on the Expi you would know right away.... It would stumble a lot and there was no random or gradual build up. It simply was done when it was done. I am assuming the GM coil packs are the same way...but this stumble I have is very slight to moderate and some days it does...and some it doesn't. I have no hesitation on acceleration and no real change in mileage or power from what I can tell.

Thanks in advance for any input.