I own a 99 GMC Z71 SLE with the 5.3 engine, and 147,453 mi. Engine began to miss, especially noticable at idle while in gear. The SES light was activated and the code read was PO307, "cylinder 7 misfire". After checking the plug, plug wire, and coil pack to this cylinder, I found it had nothing to do with "fire" at all. It was a clogged fuel injector. These injectors have a funnel shaped screen at the top. The screen was totally clogged with a rusty sediment. I carefully cleaned the screen using a toothpick and WD40. You don't want to pierce the sceen! My dealer charges 104.95 for one injector. It has run like new since, about 500 mi. ago. Also, I noticed the fuel rail had a "rust like" sediment in it. I plan to pull the fuel rail off the other bank of cylinders and clean it and the injectors as well. Thanks to the people who posted here about clogged fuel rails. You helped me diagnose this one! :)