my car runs just fine when i wake up in the morning, but about 15 miles later it starts misfiring. i took it to a machanic, and he couldnt figure it out. it idles really rough, shaking the car. i decided to try and find out which cylinder misfires by unpluging each one and seeing if the idle drops. so i found which one it is and grounded it to see if it would spark. it wouldnt. so to make sure it wasnt the black thing i switched it(not sure what its called, the thing that connects to the spark plug) with another one it still misfires, and its black thing works with any other cylinder. so it has to be the wire or what ever is sending the charge to the wire right. sorry im not a machanic obviously. also how i said it works in the morning, i tested this morning on its first start, and it was firing. i took it back to the machanic and showed him, and he thinks it must be some computer problem. please help what do you think. no check engine light comes on or anything.

 

its a 91 z32 by the way. ive only owned it about a month. it really sucks

 

Im just a noob when it comes to cars, but from an electronics perspective it sounds like you have a seperated wire that when warm the shielding stretches letting the wire seperate (EG when cold even though its broke it still touches). this is just a guess though it could be alot of things, my suggestion would be to check the wire that feed that as far back as you can bending slightly (with engine off) evey inch or so to see where it feels wrong. It may not be it at al but atleast checking the wire will elimite one possibility.

I also had a similar problem when I first bought the car, and it turns out someone had replaced the PTU and had used an adapter to make the connection work, and adding that foot or so length made it hang next to a belt which rubbed through the shielding and cut the wire causing number 5 to sometimes fire and sometimes short to number 6.