Mysterious Misfire

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chrmaka

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Okay guys, on my truck its misfiring really bad and I'm unsure why. I have good compression across the board. Twin aeromotive 340's in the tank and it's holding a consistent 62 psi of fuel pressure. There is no new codes but the misfire does not happen at idle or WOT. Has full power and idles fine. its missing at light throttle around 2500 rpm and gets worse after the motor has reached operating temp.

These were replaced last summer:
Crank sensor
fuel lines
Fuel Filter
ICM

These have been replaced within the last 3 months
Plugs
Wires
Coil
Distributor

And I just replaced the MAP and spider unit today as well, I went to a ported factory MAF that I know that works with no change as well. On the tuning side I've reverted to a tune that's as close to factory as possible (factory timing and slightly more fuel than factory) with no luck. Before I swapped the MAP and spider unit out my plugs were showing signs of detonation from the other unit going bad. If time and weather permits tomorrow I'll pull the ones I threw in today and update. Before I did all this my truck was undriveable because of the bogging, misfiring, and backfiring. Now its drivable but my cruising RPM band is missing and occasionally backfiring now.

What I need to know is if there's anything else that I could be forgetting that could cause this misfire?
 

chrmaka

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Also, I have a 6AL2 box on the truck as well, I'e tried unpluging it and doing the ignition without it and the problem still exists with no change.
 

slowburb

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You might want to run a leakdown on each cylinder to see if you have a sunken valve seat or other valve sealing problem. Are you getting a P0300 or is it calling out certain cylinders?
 

chrmaka

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No misfire codes. Only code i had was a p0103, after resetting it, it has not returned.
 

chrmaka

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Also to note, I have solid compression across the board as well.
 

chrmaka

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True. It is possible i could have had another rocker collapse now that I think about it. Looks like i'm pulling the covers off tomorrow as well, I'll know more when I pull the plugs again
 

chrmaka

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UPDATE:

Pulled the plugs, soot covered, showed signs of incomplete combustion
Passed leakdown test

Proceeded with pulling one wire off the distributor when the motor was running found inconsistant spark from cylinder to cylinder. Removed the cap to find it was already corroded beyond belief and couple posts are burnt.

Will update when I get the truck back together.
 

chrmaka

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New distributor cap got rid of the hesitation when coming out of a stop and the shudder from idle is gone. Now it's idling at 600 rpm and missing at anything under wot.
 
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