1998 Chevy Silverado Check engine light on

I have a chevy silverado 98 My check engine light is on I took it to 2 mecanics already I payed 400 to one and the other 800. The scanner keeps telling me the problem is a oxygen sensor but I changed one already the first mecanic told me that the proble was the regulator so he changed it and the other one told me that the problem are the gas injectors so he changed that and the spark plug wires too. When the light went on for the first time the car started to loos power it shackes and it smells like gas and the pipe is really black and it keeps trowing some black thing out the pipe. The first mecanic told me that he dosent know what it is the other is still working on it soposly and I dont want to keep waisting money for nothing. What can I do?