2000 suburban ground issues

[bufmatmuslepants]

I bought a 2000 suburban LT with 239k miles for $1800, extremely clean. It had a check engine light with P0332 knock sensor bank 2 low input and P0430 bank 2 catalyst system below efficiency. It also would throw LOW OIL LEVEL and LOW WASHER FLUID lights after running for a bit, and sometimes the ABS and BRAKE lights. As soon as I would shut it off then restart it, all lights go away except check engine, so I knew it was a ground issue. The engine doesn't smoke or knock, good oil pressure, oil level is fine, washer fluid level is fine.

I started with grounds, I cleaned the battery terminal ground, and for the first 10-20 minutes it would only have the check engine light, but after it got warm it would still throw the ABS and BRAKE lights. I cleaned the EBCM ground, no change. It was idling at 550rpm normally, but after disconnecting the negative battery terminal for 15 minutes while I cleaned the ground it would sputter down to 100-200rpm and took a bit for the IAC to catch up, so I cleaned the TB and it is a little better.

I put my timing light on the plugs on bank 2 and was not getting consistent spark from the passenger side second one back, so I changed the plug to a NGK platinum gapped a touch tighter at 0.055. Still not a good spark from the timing light. Then I unplugged each coil and every coil did change idle so all are firing, timing light must not be able to pick it up well.

I put a meter from battery to frame and battery to the alternator bracket where it says GND and with the engine off I got 0.3 ohms, so good, but engine on and warm I got 60-70 ohms. I cleaned the battery to block ground and the battery to frame ground and now it's 25 ohms engine on, can someone verify this is ok, or measure on their car? I feel like that is high. I moved the battery to block ground a little farther away from the block so it doesn't get as hot. I cleaned the block to firewall ground at the firewall and couldn't reach the other end at the back of the block in the dark, but still negative battery to firewall is 25 ohms running.

I ran 10 gallons of shell 93 with Lucas fuel injector cleaner the day I bought it, and now it has a full tank of Sheetz 87 with another bottle of Lucas fuel injector cleaner.

Today it made it 35 miles and 45 minutes to work without throwing any lights at all, but I don't think I am out of the woods yet. I am now leaning towards spraying brake cleaner on the intake to find a leak, and possibly replacing the intake gasket, I don't think the knock sensor is bad since the bank 2 catalyst code tells me that bank does have an issue. I have not spent a lot of time with LS engines and while it is smoother than my old tbi 350, vortec 350, or LT1, I think it should idle smoother. Other options are clean the MAF, replace the rest of the plugs and put on new wires, and buy a brand new battery negative cable. The plugs on it are AC Delco platinum with normal wear, and the wires look decent, I have all new plugs but haven't bought new wires yet.

Any ideas?
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