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Demonic misfire gm 5.3

Old 02-13-2019, 08:11 PM
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Default Demonic misfire gm 5.3

I have a 2001 GMC Sierra 1500 with the 5.3 v8 , 706 cast heads, and 4l60 trans. I am having a cylinder 5 and 6 misfire and have been for months. Despite me doing a lot of work and it being in various shops for 6 weeks, the couple of grand shocked into have gone no where. 5 and 6 have constant misses but are not dead misses. Here's a list of all parts replaced and work done
spark plugs
Plug wires
coil packs
Injectors
fuel pressure regulator
Knock sensors
Evap canister vent and solenoid
Fuel filter
Fuel pump
Air filter
o2 sensors
Intake gaskets
crank sensor relearn
Cam positioner relearn
injection cleaner
Induction cleaner
Multiple fuel pressure tests 55+ lbs while off
Ecm flash
New ecm
All valves, lifters, valve springs, rocker arms, and camshaft removed and inspected for malfunctions (none found)
head casts inspected with no blemishes etc found
all wires traced and inspected for bad grounds or malfunctions
compression test (90 or something like that)
leak down tests, passed
all gaskets and seals are good.
all ignition components and injectors have been placed at random to see if problem switches cylinders, nothing changed

Everything listed above has been done multiple times by various shops. I cant find anything online about this problem either... any help is appreciated.
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Old 02-13-2019, 09:28 PM
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What octane fuel do you use at the pump?
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Old 02-13-2019, 09:30 PM
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Originally Posted by ILuvJDM View Post
What octane fuel do you use at the pump?
87. Same I've used in it for many years.
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Old 02-13-2019, 10:21 PM
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Not easy or ideal swapping the whole harness to fuse block would be something I'd consider. Even with tracing wires it could still be a cause.
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Old 02-13-2019, 10:23 PM
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Originally Posted by FFDP View Post
Not easy or ideal swapping the whole harness to fuse block would be something I'd consider. Even with tracing wires it could still be a cause.
you're saying swap the entire fuse box wiring harness? But wouldn't it show up on when people have traced all the wires and and stuff, or how would it not show?
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Old 02-13-2019, 10:40 PM
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I'm talking the whole engine harness to fuse block. I've seen some weird things happen even if wires were traced and had good power/ground when tested.

I'm not sure where else to look. You've had every physical part replaced already but the harness.

I did a little googling as well and 90psi of compression is low. Looks like spec is 100psi minimum and depending on engine would be over 120-140psi normally.
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Old 02-14-2019, 07:03 AM
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Originally Posted by FFDP View Post
I'm talking the whole engine harness to fuse block. I've seen some weird things happen even if wires were traced and had good power/ground when tested.

I'm not sure where else to look. You've had every physical part replaced already but the harness.

I did a little googling as well and 90psi of compression is low. Looks like spec is 100psi minimum and depending on engine would be over 120-140psi normally.

I thought it was low but the shops said that's normal. But all cylinders read the same compression so I wouldn't think that would cause the misfire
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Old 02-14-2019, 08:09 AM
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So it hasn't been mentioned or asked,

Any mods?
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Old 02-14-2019, 08:10 AM
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Originally Posted by madmann26 View Post
So it hasn't been mentioned or asked,

Any mods?
Stainless headers cause the stock cast iron ones cracker somehow. Dont know how just did has a borla muffler on it. But I've had these on for a while and only recently had problems
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Old 02-14-2019, 08:58 AM
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Originally Posted by Tmulls1995 View Post
Stainless headers cause the stock cast iron ones cracker somehow. Dont know how just did has a borla muffler on it. But I've had these on for a while and only recently had problems
So,

Headers? No tune?

Vehicles are finicky to changes. Have you logged for misfires?

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