Potential Catalytic Converter Issue?- And a Reintroduction

[LJ11194]

I used to be on this forum a lot several years ago, but I found myself in a position where I did not have enough experience or “mechanical inclination” to act on the advice given to me here. I often lacked the tools to properly figure out what was wrong, or to fix it. I’m pretty sure I didn’t even own a jack the last time I posted here- so my ability to diagnose or perform repairs was extremely limited. Over the last few years I’ve acquired many tools, more space to work, and completely different vehicles than what I had before. And I’ve developed some ability to properly diagnose and fix problems on my own- which I severely lacked before.

So with all of that out of the way, this is going to be a long explanation of one of my vehicles and what has gone on with it lately. I apologize for the long windedness but I need to make sure I explain what all has happened in order for any diagnostics to be well-informed.

I’ve been daily driving an 05 Suburban 2500 6.0L for the last few years. I’m up to 170k miles but it is in excellent shape- it’s cleaner underneath than some 5 year old trucks- and it’s spent it’s whole life in Pennsylvania which is outstanding. A couple years ago I had the rockers and wheel arches replaced and repainted because they were just starting to rot, but due to the mechanical condition I felt the investment in the body was warranted. It’s not my daily anymore, but when I need it, I really NEED it either for the space, the 4×4, or both. The reliability has been great, I put 20,000 miles on it last year. Around 7,000 miles ago it needed a fuel pump.

About 2,000 miles ago the exhaust sound completely changed. At this time it was still running just fine, but it started sounding restricted and much quieter. I have a flowmaster 50 muffler which is not very loud, but it became suspiciously quiet. I knew something was wrong and I suspected a plugged cat. There were no codes but one of them failed the temperature test. I took it down to a local muffler shop and they discovered that many of my exhaust manifold bolts were broken and according to them, it was “sucking in cold air into the exhaust.” Not uncommon on LS based engines, so my friend (who is a welder) and I removed the broken off bolts and replaced them with studs. I removed a the plugs in the course of our work and discovered the center electrode on all eight was worn down to the ceramic. These were AC Delco iridium plugs with about 70k miles on them. (I thought they lasted longer than that?) The gap had increased to .096! I’ve never seen this before, neither had he. I promptly replaced them and we put everything back together and the drivability problems began. It will occasionally slightly shake or run rough when idling, cruising, or accelerating but it’s random. I also have a lack of power at low RPMs (it won’t creep forward/reverse under idle power), but if you get it above 3,000 rpm it runs great. At those engine speeds there is a pronounced air rushing type noise from the exhaust rather than the deep growl it should have. I had it stumble and almost die one time but that seems to have been an isolated incident. The exhaust still sounds strange, instead of the smooth V8 rumble you expect, it makes a constant sound like an exhaust leak- rough, metal rattling sound. The noise is most definitely coming from the tailpipe; I cannot find any other leaks anywhere in the exhaust system at all. I still have no codes, and one of my cats still fails the temperature test. I have included links to videos of putting a vacuum gauge on the intake, showing fuel trims, showing 02 sensor voltages, and the catalytic converter temperature test I did. I tapped on them both and the muffler with a rubber mallet and don’t hear any sound that would indicate something came apart. Note that I had to do the cat temp test at idle instead of 2000 rpm as Eric showed in a video because I don’t have a helper.

I towed a fairly light trailer (<3000 lbs) a little bit ago and while it had the power to pull it at higher RPMs, it was NOT HAPPY. It ran extremely rough after working that hard (although I only drove 15 miles) and the sound from the exhaust at idle sounded like an aggressive cam or a severe misfire- but no codes.

TLDR: 6.0L Vortec, 170k miles. Absolutely huge exhaust leak at manifolds was fixed, spark plugs’ center electrodes were wearing away and were replaced, exhaust doesn’t sound normal- sounds like it has a leak but doesn’t, lack of power on low end, rough running comes and goes, worse if you try to make it do any real work.

One of two cats fails temperature test, vacuum test passes, fuel trims and 02 look OK to me but I need more guidance there.

Vacuum test-

Fuel Trims-

Cat Temperature-

If you read all of that, THANK YOU! If anybody can give me any other avenues to research before I buy a cat, I’d greatly appreciate it! The heat test and vacuum test contradict each other- Would an exhaust backpressure test definitively prove this? I’ll have to buy a gauge for this test.

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