My Dad has a 2000 Hyundai Elantra, the engine runs rough and the fault code is a misfire on cylinder number 4. He had me change the plugs, I did a compression test, the cylinder had 130 psi pressure, I connected another plug to that cylinder and the spark seemed hot, I tested the fuel injector plug with a test light and it seemed to switch properly. I figured it was a clogged injector so I replaced the injector and there is no difference. Does anyonehave any idea what is going on?

 

Sounds like the mechanical aspect is ok. How about the source that triggers the spark. I take it you checked the ignition wires. Can you get a flow chart that shows the ignition components and see what triggers the ignition, i.e. crank position sensor, etc. If it is cosistenly cylider #4, look for a shorted wire or one with a worn isulator ans grounding out the ignition. Did it run ok before and start doing this on it's own or coinside with work done on the car. Some plugs are not compatible with certain motors and should use the original stock plugs. A plug that fires out of the cylinder may not in it. The plug has to deal with a lot of different conditions to fire in the cylinder.

If you have a vacuum leak in the intake for that cylinder and the air/fuel mixture is too lean, it wont fire in the car.
Hook up a vacuum gauge and see what readings you get. If you log on to HMAservice.com, it has a section, animated, on vacuum rading. That web deals with new models, but vacuum readings will give the same indications for new or older motors

 

"I take it you checked the ignition wires."

You know, I thought I checked the wires, I inserted another plug into the #4 wire and laid it on top of the head, it had a nice hot spark, so I thought the wire was good. It never occurred to me the insulation could be breaking down, especially in the boot as I never had that happen before. As it turns out, it was arcing under the dust cover so you'd never see it. Thanks for your advise, I really appreciate it, I had been searching the internet for a few days now and you're the first to suggest it could be the wires.