I Had this same problem with my 93 Jimmy last month.
I would try removing the plenum and looking inside the intake. On mine the fuel line inside the plenum feeding the CPI was cracked causing excess fuel to fill my intake and pour down the cylinders. This also filled my oil with gas as well as broke my starter trying to turn it over when the cylinder was full of fuel. Lol
I replaced the fuel feed line and also the return line inside the intake. Drained the oil, replaced oil filter and oil, then ran for 10 min then drained the oil and replaced the filter and oil again just to be safe and have put over 1k on mine since then and it has been fine.
Also mine had been hard to start for a month prior. This was the warning sign but I didn't listen. Now it starts like new ;) 225k and still going!
I had also read that the cpi can go out and cause it to add excess fuel but that wasn't the case for me.
Monday, August 18th, 2008 AT 3:32 AM