Classic/Antique Car Repair: Chevrolet C10 Deluxe 1985

Question
Fuel pump (2nd time) keeps mixing the oil with the gasoline and wont stay on.
What could cause this, could it be dirt in the gas tank?

Answer
This truck when built was equipped with a Rochester 4 barrel carburetor and a mechanical fuel pump. The first place that I would check would be the fuel pump. Often when the diaphragm in the fuel pump ruptures it will pump fuel back into the crankcase. Once that happens then gasoline vapors from the crankcase will be drawn up through the Positive Crankcase (PCV) Vent valve and cause the fuel mixture in the engine to rum rich. This can cause rough running and stalling at idle. If the fuel pump is OK then the problem could be a leaky fuel inlet needle in the carburetor of a sinking float. But the over flow on the carburetor would have to pretty severe to cause dilution of the engine oil. My bet is on the fuel pump.