found this discussion on reddit:

[–]nibbles200 1 point 1 month ago

I am very familiar with the 5.7 vortec that is in your truck. Very common to get missfires when the intake gasket is failing which is a real weak point in these engines. Here is what you do, pull all the hoses off both valve covers and plug them, like with ductape or if you have a few hands, hands. The proper way to test this is with a vacuum gauge with a host squished into the dipstick hole and any vacuum indicates an intake gasket leak, you should have a slight pulse or a little pressure, not vacuum. when I plug all the valve cover holes, if I feel a lot of vacuum I usually don't bother with the vacuum gauge and just assume intake gasket.

The problem with the intake gasket is that they are made of plastic and warp over time until they fail. There is a newer design where there is a piece of sheet metal inside the plastic gasket and it seems to be a more permanent fix.

Other possibilities. The injectors on these truck are another weak spot. They are a open @ yield poppet valve and tend to get gummed up and sticky. I found running a ton of sea foam can help if this is actually the problem.

Other known weak spots: the distributor wearing out. If you pull the distributor cap and can wiggle the rotor or see any damage to the cap pins then time for a new distributor.

Last possible know issue, The distributor caps on these are known weak spots due to their flat design they can fail and short across so replace this regardless and check the distributor while you are at it.

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[–]ansag1[S] 1 point 29 days ago

after all of the over analyzing and tearing down the truck, it all turned out to be a weak distributor cap. It tested with an ohm meter and all was well, but it wouldn't pass enough voltage to arc.

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[–]nibbles200 1 point 29 days ago

I probably should have listed that in reverse, start with the quick cheap fixes first. Happy to hear you are back in business.

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[–]ltmoto 1 point 1 day ago

as a tech when i get a misfire car in i start buy doing a relative compression test to check the mechanical integrity of the cylinders, so i know if i need to diag mechanical or electrical problem. normal compression test isnt the most accurate test.

seems like you have covered the electrical part, maybe not the injectors because they are a pain. but if you want to you can remove the upper intake and remove the "spider" assembly and swap 6 with another injector, reinstall and see if miss changed to swapped cylinder.

back on the mechanical side with that engine valve seat problems are common with im guessing around 200k miles on your vehicle it will probably needs a valve job.

at my shop this could of been diaged for about 120. you would know right away what the problem is and how much it would be to fix it. from your list of parts it looks like you spent 5-600 if they were quality and not napa "specials" . had to put that in for all the "parts throwers" out there. better to get it figured out before you put so much money into it that you cant afford to not fix it.

i know everyone has a bad experience with shops but not all are bad. hope this helps

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