I have a 1996 chevy with 5.7 vortex codes say cylinder 3 miss fire but I've changed plug, wires, coil pack, distributor cap, and button, even whole distributor. i've asked around sand some people tell me it just a bad cap that they are a bad design some say its injectors or burnt valve I've been dealing with this issue for 4 years I've chased everything and no conclusion. one of my coworkers said he had same issue, and it was a fuse under the hood he can't remember which one though. it's weird though sometimes it drives perfectly fine others (majority of the time) is runs like its down a cylinder. it's like if I start it cold and drive it drives down a cylinder and once it warms up and i shut it off let it sit for about 30-60 mins and start it again it'll run good for about 15-20 mins then go back to running like poop. idk my mom's mechanic bf keeps telling me it the cap, but I shouldn't have to change the cap every week you know. so, someone please help me!! P.S. truck has 308k on body, but crate engine was put in at 242k.

 

Check your compression on cyl 3. Could be a valve that’s acting up too.

 

A compression test will tell you a lot. If you don’t have great compression, but some, it’s probably a valve. Get the #3 cylinder to its tdc. Where both valves should be closed and pressurize the cylinder with air. See if you can’t hear where the air is escaping. If it’s up the intake you have a problem with your intake valve, or if it’s escaping the exhaust it’s an exhaust valve issue. Your Cam could be going out as well and your valves aren’t working correctly.


Have you checked your timing?

 

timing is good yes. I've checked compression before I don't remember the numbers, but it was good according to a family mechanic. I've also did the trick to spray brake cleaner around the intake manifold to make sure the gasket isn't bad and that was okay.