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.

 

You need to check your cylinder pressures. Run a good OBD II scan on it. The GM caps are made good unless you bought a defective one made in Asia. Clean your injectors with PEA. Techron Complete or Lucas both have PEA. Run it hard with a Italian tune-up on some safe road and do not fill up until your gauge is near empty.

 

I've done a pressure test and had codes ran before. pressure was good popped up multiple codes like miss fire on 3 O2 sensor(has no convertor so most likely the reason for code) and i think it was mass air flow sensor or something. I've also ran 93 and PEA though it before didn't help.

 

measure your fuel pressure

 

if my fuel pressure is bad would that be bad injector?

 

if the pressure doesn't build up to spec, then it's a bad pump.
If pressure comes up good, but then bleeds down, it could be a bad injector, or bad pump. You would need to install a valve between them to make that determination.

 

one thing that can cause "random running goodly and poorly" is intermittent oxygen sensors. they don't always throw a code and when they create a stumble and running rough the PCM will throw a misfire. this is a niche condition i am not saying it is your solution. but it is a hard condition to find since most DMM cannot measure O2 sensor continually or at all

 

try unplugging BOTH THE FUEL AND SPARK to the cyclinder mis-firing. see if the car runs fine without it. that will help you exclude where the problem lies.

obviously if your problem is1 cyl it's a whole different story than if the problem is randomly on any cyl.

if 1 cyl it could be a gasket leak bad injector or what on that cyl only - and it's not an airflow issue (not egr or intake leak on all cyl). ie, radiator fluid is entering the cyclinder. if so, you can drain your oil in a clean pan, check to see if it looks bubbly or milky (in places). if your often putting in radiator fluid that also can be an indicator (note if you overfill the COLD line, it'll always spill out when driving - that doesn't count as consuming fluid). you would, if 1 cyl is issue only, remove the plug look inside. see if anything unusual (ie, very clean means coolant is getting in. you can use a pressure test to check compression. and so on.). actually first you'd check spark and then injector. not easy to swap injectors on that model however - so you'd have to contrive a way to get the injector to spray with engine off (key-on) perhaps. just fuel smell or drip would be telling enough. (injectors can be intermittent when failing - also injectors can get clogged but can be cleaned if on a cyl with fluid barrier leak)

if on all cyl then it could be allota things so i won't comment on that (yet)

anyway i'm quite sure you should disable that cylinder and drive it and see how it goes

 

"burnt valve" sounds drastic and typical of problems in "much older vehicles that weren't well maintained"; that would be rare. broken spring or loose lifter more likely but you'd hear that. exhaust leak - you can check for.

and the paper trick. put a piece of paper on your muffler. see if it "pushes and pulls it regularly". if it doesn't it could indicate value (maybe). and if you have smoke it could indicated valves (or rings, depends)

your right in being suspicious of advice you heard. always go on what you've excluded scientifically. bench test the rest. never go on guesses.

 

PSI pressure should be at 56 to 62 PSI.