coolant in oil
If it’s going directly into the oil pan you have a busted block. 91 Nissan is not worth much, but if your in love with it get one of those imported used engines, I’ve used a few of them and they are fairly good.
There isn't a connection between cooling system and engine that would transfer coolant that fast. Are you sure you're not putting coolant into the engine, in stead? Only other thing it could be is a massive engine block crack at the back of the water pump, but if you had this, the engine probably wouldn't run. The only car I can think of that would cause your problem is the old Lincolns that had their water pumps inside the engine block.
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