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tech help. 4.8l misfire

bikrboy128

Some say.....
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Jul 15, 2007
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just throwing it out there for all you lsx gurus. hopefully someone can help.
ive got a thorn in my side at work. 06 chevy pickup with a 4.8l has constant misfire on cyl 4 at idle. it has about 80k miles and is used by a company as a work truck. the truck runs great off idle, and the cylinder doesnt seem to be "dead". just watching my scan tool it misfires about once every 2 seconds.
anyway i pull the plug and it's worn. replaced plugs and the misfire is there still. switch coil and wire with a different cyl, no change. the injector ohmed within spec. sprayed for vacuum leaks, found a small leak on the intake gasket. i replaced gaskets and injector o-rings, along with moving cyl 4 injector to a different cylinder just for shits.....everything got put back together and torqued to spec and cylinder 4 is still missing away.
the engine sounds strong (no knocks or funny noises), fuel trims are +/-4 on both banks. O2 readings look good. i cleaned the MAF then unplugged it. i went as far as injecting smoke into the vac lines to see if the plenum itself was leaking, but didnt see anything. i'm stumped and not really sure where to go next. soooo any help would be greatly appreciated.
 

Eagle

Nemo me impune lacessit
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Mar 1, 2008
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Woodsticks, IL
1. First guess is wiring / electrical gremlins. Check the actual connectors to #4s inj and coil. Ohm out all wires back to pcm from same components.

2. If you have access to a spare, Pop in a replacement pcm and see if it changes. Trust me on that one... I've seen it personally.
 

02BlueGT

No Fucks Have Been Given
Feb 21, 2008
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Now apearing in Hanover Park
Run compression/Leak down test? a slightly bent/damaged/leaking valve would cause a misfire like that, also look at control circuits, for both the coil and injector, without the correct signal/voltage they will not work properly.

And for the Record, Ohm'ing an injector only verifies the coil is intact, it can still be stuck or leaky with good ohm numbers (I know you moved it, and it isn't the problem)
 

bikrboy128

Some say.....
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Jul 15, 2007
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I did an injector balance test and the fuel pressure drops the same across all 8 cylinders. So that tells me the injector and wiring is good. Right?
Now here's the shit...Did a leakdown and compression test this morning. Leakdown was good, but compression was 90 PSI while the others were just above 120. :fu:
 

bikrboy128

Some say.....
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Jul 15, 2007
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Wheaton
also, no spare PCM. im going to be checking wiring and voltages at the coil.
i did pull out my old snap on DIS kv probe, and all cylinders have good spark output. also, if i raise the idle from 550 to 700 rpm the misfire goes away. it's not looking too good right now though. :hs:
 

bikrboy128

Some say.....
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Jul 15, 2007
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Wheaton
Stupid question..but you didn't say anything about it. I've traced many LS motor misfires to plug wires that Ohm perfect but misfire under Idle. Did you check the wires for cracks and or arcing at night? I've seen this a lot.

i did check wires by spraying them with water and watching for fireworks...nothing. i also tried moving them to a different cylinder and watching the misfire counter on the data stream. no change at all.

the truck is gone thankfully. the company didnt want to go any further into the problem, and opted to raise the idle. raised it to 750RPM and the misfire is gone and engine light stays off.
i really hate calling this "fixed", because it isnt. but the customer declined to go deeper into it. oh well :dunno:
 
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