i couldnt find anyone that has had the same problem as me on here so this i my situation... i have this random misfire that occurs on my 96 Z28. it only does the misfire after the car has warmed up (no misfire at cold start). i have been to hell and back trying to figure this out and its been going on for 2 months. it is most noticeable during heavy load usually between 1000 and 3000 rpms, and anytime during quick acceleration. i have replaced the coil, ignition control module, opti spark, IAT sensor. Has brand new plugs and wires. took it to a shop and they told me that number 5 injector was bad so i replaced it and it didnt fix it. so took my friends 95 firebird formula fuel rail and all 8 injectors and put them on my car and still misfire. also switched out the MAF and that didnt change anything. also put new O2 sensors on... so i dont know if i missed something or what but i would appreciate any help that anyone can give me.

 

Do you have an OTVC Kit? Are your plug wires coming in contact with your headers? Did you do the plugs yourself?

 

no i dont have an otvc, im not even sure what that is. i had a shop do my plugs and wires. i burned through one set of wires and had those replace and all the wires are ran and zip tied so they dont touch the headers.

 

You might need to check your plugs first, I was told a shop did mine and when I pulled them 3 of the plugs were cracked. So thats where I would start. otvc is a kit to run your plug wire over your valve covers to keep them from getting burned. Are you getting a SES light?

 

no engine light. only as it gets hotter the misfire gets worse and when it gets bad enough the engine light kicks and throws a random misfire code. but as soon as it cools back down the light goes off and it doesnt misfire as bad.

 

I had a random misfire and finally traced it down to a missing platinum puck on a new plug. In my case the miss counts started piling up on cylinder #3 which was where the plug in question was. Your miss counts showing preference to any particular cylinder or cylinders?

 

im not exactly sure which cylinder it is. im guess since the shop told me number 5 injector was having problems thats the cylinder thats missing the most. i am tempted to go and get my own tool so i can do all these things myself.

 

Now for some reason the misfire has gotten worse and the random misfire code has popped up and is staying up. But still only is misfiring after my engine is warm. VERY FRUSTRATING! I don’t know if this is just coincidence or if it is because I have been messing with things… so tomorrow being my day off I will take my car into my High School auto shop and change out and check all the plugs.
Also I just bought some taylor high temp 10.4mm wires and I will put them on as soon as I get them. I have heard good and bad about these wires but they were the only ones for some reason that I have been able to find that can stand up to the high temps of header(supposedly).

 

have you checked the injector pigtail on #5 to see if it was dirty or corroded, also check your wiring harness by your steering shaft sometimes it starts to chafe there,

on second thought maybe u have a lifter collapsing on a cylinder or a lobe getting worn down all it takes is one lil burr to ruin a cam and cause a missfire

 

would the cam only miss under heat though? i thought heat was more of an electrical issue. so maybe the wires to the injectors could be worn and rubbed through to. tomorrow i will be checking all of that when i have access to the drive on lift.

 

Well, I too am having somewhat the same problem as you. I have a 97 Z28. Hooker Headers, Msd everything, Flowmaster exhaust, BBK Throttle Body, K&N cold air. She runs great off of a cold start, but once she warms up a bit it starts to miss. i have a scan tool and my biggest miss is on cylinder seven. It's getting a little late in the season, so i'm not really that concerned, i will work on it in the spring, but previously, I had taken the car to a 1/4 mile strip and it ran 17's which is absolutely terrible. i found that the throttle body gasket was leaking which wound up making it pump too much fuel in and fouling out all of my plugs. also my timing cover was leaking onto my msd optispartk and soaked it, so i put new plugs and wires in, and put a new cap and rotor on. replaced the gasket and it is running better, but still missing. i do not have a catalytic convertor. i can't remember what my codes were, but most of them had to do with the IAT sensor. i have dummy downstream o2 sensors. if i figure anything out i'll let you know, or if you have anything new, let me know

 

so i re read your symptoms and came to the conclusion that u have a possible blown head gasket on the side that your getting the missfire , see when the engine is cool there every thing is pretty much tight and contracted then when the engine heats up and the cooling system pressurizes and now all the metal has expanded the head has probably lifted a lil off the block and now the presurized coolant can now enter the cylinder and cause the missfire, thats why it only comes up when hot , it only takes a tiny bit of coolant to cause a missfire, thats probably why when u swaped out injectors that it didnt help, so it kinda narrows it down to a blown head gasket,or the ign system, or a faulty pigtail on that inj on that cylinder, or also have u checked to see if that plug is cracked?
or swapped that cylinders plug wire with a known good one to see if it goes away?

but also a lifter may also not start collapsing till the engine has heated up and the oil has thinned out , those are the thinks i would look into ,