The trucks been parked for the past eight months waitin on a tranny and what not. Got'er going and now it sorta sounds like a flat head or something's muffled a bit. Dunno just don't sound right. Almost like a misfire when driving but looks and sounds fine at idle. I checked the temps on both cats this afternoon and one is at 260ish and the others barely at 200 degrees Celsius.

 

Best way to test for a plugged cat is a backpressure test.

 

Is the fuel 8 months old and without a stabilizer?
I don't know how the states is but around here after about 3-4 months of fuel being stored its garbage and the engine runs terrible. Misses. Stumbles. Hesitated.
When heat testing your called its not just the cat temp to check. What you should do is a delta test. Meaning when the cat is working properly its actually hotter on the way out. Usually like 50 celsius hotter out the exit due to the oxidizing in the cat

 

Yeah the fuels that old, It wasn't supposed to be down that long and by the time I realized it, it was too late. When I temped the cat I just checked it near the middle.

 

I took out each front o2 separately and there is no backpressure, so I guess the cats are fine. Only 110000kms.

 

I have it hooked up to hptuners and did a scan with my misfire cfg and it showes multiple misfires in cylinders 4,5,6. I just replaced the fuel filter and checked the plugs. So one by one to disconnected each coil pack at the harness.
1 missfired 8
2 missfired 6
3 missfired 3 and sometimes 1
4 missfired 4+3
5 missfired 4+5 and sometimes 6
6 missfired 6 and sometimes 5
7 missfired 2+7
8 missfired sometimes 7+6

Any idears? I had a p0300 code but once cleared it hasn't come back but stumbles so bad to the point its not driveable. Also for the current cylinder firing its random and I don't recall seeing cylinder 8. That was the same as last year when I had a bad coil.

 

Did you change the fuel?

 

Yeah I ran the tank down, added fresh fuel and octane booster.

 

I'm on mobile so I can't see a sig.
What year is the truck?
Those misfires are looking like the spider injectors failing. Unless its not that style
Have you pulled any plugs? They could be fouled from the bad fuel

 

Here's my sig.

2000 silverado 5.3 Z71 RCSB 4X4, 2005 caddy conversion, Black Panther 4L60E, 6" BDS lift, 3" body lift, torsion bar keys, 36" super swamper TSL SX, throttlebody spacer, 2800 stall, headers, 3" magnaflow hi flow cats and mufflers, lingenfelter high volume oil pump, double roller timing chain, comp cams 265 extreme, GMPP yellow springs, chromolly pushrods, 44lb. injectors, ported 317 heads, e-fans, Intercooled Powerdyne BD-11A, HP Tuners, Detroit tru-trac.

I have the marine injectors if that helps. How can I test the injector? Later today I'm going to fire it up and bull the power to each injector and listen for a change. I looked at the plugs yesterday and they're a tan color.