How do I test for engine misfire with Infiniti J30?

While engine is running, pull each plug wire off the plug one at a time, replacing the plug wire afterwards. If the missfire gets worse while its wire is off, it's NOT that plug or cylinder. Replace the wire and go to the next plug wire.. If there is no difference in missfire when pulling off a plug wire, it's that plug or cylinder that's bad. Caution: You can get quite a jolt when pulling off spark wires on a running engine if you short yourself out. Use a plug wire tool or wear rubber gloves, or have the wife test the wires while you sit in the car. :-)

That method only works when the bad plug is misfiring full time. If it only misfires every few cycles, you need to alter the test. Here's what to do: Pull them one at a time. They should all make the car run worse since you are pulling a plug out of the circuit. However, when you pull a good one, you'll have a new miss along with the old occasional miss. When you pull the bad one, you'll only have one miss - not two misses. Test a couple times to be sure.

First though, I'd check the computer. If it's missing that badly you may have a code in there that will tell you which cyl is off.

If you have a non-contact infrared thermometer, you can point it at the exhaust manifold port for each cylinder. The colder one is the misfiring one.