1993 S10 Blazer mileage: 175,000. I am having a rough idle, rich mixture, smell from the exhaust and black carbon coming from the exhaust as well, and I have to crank it for a while and hold the gas pedal to the floor for it to start when it is cold. When it is just shut off for a minute or so and I restart it, it will start immediately. I have put in a new fuel filter, new spark plugs and wires, distributor cap and rotor, O2 sensor, and EGR valve. I am not getting any codes CONSISTENTLY. A couple of times I got codes, one was MAP sensor/EGR valve. The other was EGR valve / O2 sensor. But the codes weren't back to back and they went away shortly after they came on.
Check to see if the fuel pressure regulator has a ruptured diaphragm allowing raw gas into the intake manifold. If you have a faulty regulator, replace it and the O2 sensor. Also, check to see if you have a partially plugged catalytic converter.