1995 Gmc K1500 Runs Rich, Please Help...

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Hoping someone can help me. I have a 1995 GMC K1500 Ext cab pickup. It is actually called a "Z71". I never seem to find this when trying to reference for parts. The truck was sold to me second hand with 190K miles. It has been maintained immaculately by my friends father who sold it to me (service records, etc). I recently changed to plugs cuase they were about 90K on them and the engine was clearly fouling on some cylinders. Two of the plugs were completely caked with some sort of sludge. This solved that problem. However, The truck runs extremely rich. My friends told me that i about gassed them out with exhuast fumes when following me in their car. The exhaust is extremely rich with fuel smell, specifically unburned gasoline. I have done the basic stuff (air filter, plugs, checked for obvious problems like loose vacuum lines. I don't know where to go from here. I I know this vehicle is fairly computerized. The problem does seem to vary from time to time. I think this engine has some kind of throttle body injection. Could it be a choke issue?? but the gas seems heavier than that. Could failed O2 sensors cause this problem?? Sorry if i have not provided enough information. I would be happy to give more. Any help would be GREATLY appreciated. Thanks in advance.