4.0 running BAD

I'm probably going to be having a look at the TPS and maybe have the throttle cleaned but lately my 87 has been running like crap. It seems a bit under powerd and the exhaust pipe is jet black on the inside. Its not burning oil (no blue smoke and the Oil level is good) so I figure its running rich. Could the coolant temp sensor (the one on the block on the manifold side of the engine) on the Renix cause this. Right now its idleing at 500 rpm but its missing and wanting to stall. Sometimes when I first start it up it will stall after a few seconds. Soon I'm going to pull the plugs and check if any are bridged (had that happen before and caused simular symptoms).

well I have some light to shed, it looks like someone has taken the gas cap from my truck. Has anyone heard of the gas cap just coming off because it was securly put on last time I filled up. I guess right now the reason its been running so bad is because its missing the cap.