runs rough at idle

have the cat checked. may be restricted causing a missfire and the manifold issue. common issue with the cat clogging. Roy

I'm sorry I am no Chevy expert. Is the Check Engine/Service Engine Soon light on, if it is information is stored in the computer of your car that would aid in figuring out whats going on. A professional grade scan tool has freeze frame information and enables you to look at sensor data with out doing intrusive testing at each sensor. Black fuel exiting the tail pipe and a red header runner sounds like unburned fuel or fuel unable to be burned. Have you checked compression, looked for intake manifold leaks. You've covered ignition and other things. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info or there is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You can get a one week subscription or year subscription very cheap. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual try http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription. Alldata is a great resource as well.

could be clogged intake manifold,check egr