Most likely the spider injectors are leaking down, which is part of the pressure regulator. See it all the time at the shopAnswerHere is a reply to another post which is an explanation of the Multiport Fuel Injection used in Mid 90's GMC Jimmy's. It's not an answer but is an explanation of what might be a problem. Read on, see ya at the bottom.
Early 90's Jimmys has a multiport fuel injection system which used a fuel regulator and delivery system inside the intake maniford. The Central Port Injector(CPI or Spider) has had a tendency to leak from the regulator putting excess fuel into one side of the intake manifold. The single O2 sensor reading the combined exhaust trys to lean the mixture by shortening up the "on" time for all of the injectors which leans out the other bank of clynders. In essence you have three clynders running rich, three running lean. Not the best situation for a stable idle. The indication of this is that one side of the intake manifold will be relatively clean(usually a yellowish brown)from being cleaned by the gasolene, and the other side dirty from the blow-by products from the crankcase vent. Check out: http://www.asashop.org/autoinc/feb2000/techtotech.htm for a expert diagnosis from an ASE Certified Tech with lots of pictures of what I was trying to explane. Look for "Jimmy CPI Unit" on EBay, they have some of the newer modified units for fairly cheap ($200), their not hard to change, just be careful not to get any dirt into the injectors when you remove the poppet valves at the end of each feed tube. That's about it if it's the CPI, hope it helps. 94
Now that said, in the olden days I would have said that you had a perferated diaphragm in the engine mounted fuel pump, but your fuel pump is way back in the gas tank. The closest contact between fuel and oil is in the intake manifold and combustion chambers. But I don't know how enough fuel could get anywhere there that would allow the engine to run. Take a look at the intake manifold as described above if you have a leaking CPI it could be dumping fuel into a cylinder and getting past the rings, if this is happening it would scrub the oil off the cylinder walls and your engine is toast.Good Luck94
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