2002 Tahoe 5.3L questions and observations

Howdy!

Vehicle is a 2002 Tahoe with a 5.3L LM7 Flex Fuel motor with 240K miles. I bought it around 2010 with 140K on it and may have replaced the plugs. I don't recall doing the wires. The plugs always look good and are gapped at .040. The plugs were recently checked/re-gapped and the throttle body, IAC, and MAF were cleaned. The truck has no factory EGR gear and is totally stock except for the PCM tune.
Shortly after purchase, the truck's Torque Management kicked in while I was leaving a parking lot onto a highway and I almost got creamed, so I got a mail-order tune by BlackBear (worth every penny to me) which led me to getting a cheapo scanner for my use later on. The intake gaskets were replaced at that time at the advice of Justin at BlackBear when he saw the OBD data I sent back. The truck runs well but often stalled in extreme hot weather during a vacation trip through Texas with the A/C on. I suspect a failing crank sensor but didn't have any test gear with me to verify. The fan clutched was replaced as it became gutless in the heat with aftermarket and that seemed to help but it's so noisy that I put the old one back on while in Michigan where I never have that problem.

Tools used: Elm327 clone adapters (Bluetooth and WiFi), Torque Android app, and a Hantek 1008 USB automotive scope.

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One day while attempting to track down a mildly rough idle along with assorted noises and vibrations I came across some oddities. At first I tried to get a fuel pressure reading but the (bleeping) nub that presses on the Schrader valve fell out of my hose sometime after the last time I used the gauge.
Next I did a secondary ignition check and found some weirdness. The odd momentary trace was most likely due to wriggling the probe clamp to seat better around a wire.

Bank 1 (driver's side) - Typical



Bank 2 - Typical



Here's a live trace if needed:


Note the burn trace riding higher up the line. Maybe one bank was running leaner than the other as I can't imagine I got the gap wrong on 4 plugs. For giggles I swapped a low and high wire between banks and the trace followed the wires. Okay... for more giggles I tested the wires with a DMM and got consistent readings of 1K ohms. The motor has individual coils and all the wires are identical and about 9 inches long. Granted, a low voltage check is inconclusive on HV wires but I would have thought to see some variation. :whistle:

Why the "wear" difference between banks? Are the wires worn out? Is there an optimum or preferred voltage range the plugs should fire?

I also noted that my idle short term trims bounce around -2 to -3. Would it be safe to assume that's due to the PCM tuning to prevent hesitation from a stop?

Watching my O2 sensors I see that the downstream pair bounces along with the upstream. When I raise the RPM or do a snap the downstream may flatten out or not according to some Cosmic Rhythm that has little correspondence with where the throttle is at the time. I am sure the cats are hardly functioning, if at all, due to age but they don't seem plugged. I vaguely remember replacing an O2 with a Bosch unit many moons ago but can't recall which one. I will do a back pressure test soon and work out how to capture traces from the Android or otherwise get a live graph posted if anyone is interested in hashing it out with me as I am unsure what I should be looking for.

On a good note, I found one of the noises and vibrations that was bugging me. The PCV valve started buzzing and shaking pretty loudly. Cleaning didn't help much and since I was getting the sudden oil consumption issue I replaced it with a fixed orifice type per TSB. The idle smoothed out a bit after that.

I also get an odd exhaust rhythm that I don't know whether to be concerned about or not. It's like 1 cylinder is louder than the rest. I just assume a modern (to me, anyway) MFI motor should be more even.



Anyway, sorry for the novel. I'm just trying to be thorough with the info and history.

Grouchy

Hey Eddie! Thanks for providing the captures and video, super nice to have.

The exhaust sound... I definitely hear what you mean, a rhythmic 'thump thump thump' at idle. Honestly, though, this sounds somewhat typical of GM 5.3L's to me *shrugs* There are some old school tests, like the dollar bill over the tailpipe trick, that potentially point to a valve sealing problem. Honestly, though, I've tried it before, and never found that it conclusively pointed to a mechanical problem.

I am REALLY interested to see what you found regarding the secondary ignition trace! I've witnessed the same thing when doing testing, but never connected the burn line riding higher up to anything, and just chalked it up to my scope or test leads. To me, you've got enough evidence to replace the wires, and the plugs if they've got 100K miles on them. I wish I knew what caused that phenomenon :blink: If you do change wires, let us know if the 'high' burn line comes back?

Your short term trims look great to me, how were the long terms? If you don't see a big difference from bank to bank, then I'd say you're in good fuel control overall, and that the wire situation isn't causing any kind of lean condition.