So, this may be a long one but I'm going to try to keep it in a nutshell and not leave out any details. I've been fighting with this thing for 4 weeks now and I'm getting desperate.

I tried helping this friend of mine with his 2015 Silverado 1500 with the 4.3L LV3. He claimed he had it at 2 other shops. One of them quoted him for a engine replacement, the other didn't get to finish their diagnosis before he pulled it from them and decided throw parts at it himself. He put all new coils, plugs, wires. Threw it back together and it started once and ran like shit for about 5 seconds and died. and would not start back up. When he cranked it for me you could tell it was missing compression on more than 1 cylinder. This is when he had me help him with the diag.

First thing I did was tested compression on all 6 cylinders. We had 165-180psi all on the passenger side bank, however driver's side bank had 60psi on 1 & 5 and 120 on #3. Did a leakdown test with 80% loss, and pressure coming out the dipstick tube.

At this point I told him he would be best off replacing the engine with a remanufactured or a used one, he opted to go with used to save money. Ended up finding one with 102K miles for 3 grand. I asked them for pictures, made sure the vehicle hadn't been wrecked in the front, they claimed it ran good before they pulled it.

Now we have the engine in and it ran really well the first time we started it, we had to chase down a rattle noise from loose pieces of catalytic converter in the exhaust. Got the crap out and now it runs good sometimes, and other times will bog down and shut off, and yesterday it got so bad it would start and shut off immediately. I have absolutely no codes and live data is sending me on a wild goose chase. When its reallly running like crap the long and short term fuel trims will peg down to -100 on both banks, other times it will run better, but still be shaky and the trims will bounce around, but seems to consistently subtract fuel, ranging from -20 to -4

Another thing to note is I stuck a camera into both upstream o2 sensor holes and found the passenger side converter substrate is in tact, however with a large crack down the middle, but the driver's side converter is completely missing the honeycomb material. Checked for backpressure in exhaust and found nothing.

I'm happy to answer any questions and will update on any details I may have forgotten. Thank you!