1992 Mitsubishi Galant black smoke out exhaust, gas in oil

Auction salvage car, with burnt back seat, melted rubber mount that attaches exhaust pipe to body, directly under burnt back seat, found gas in oil, no gas in vacumn line coming from fuel regulator on fuel rail, all spark plugs very black. At first, I thought a lit cigar caught the seat on fire, (ha-ha).

now I suspect clogged, caty. Convertor or bad oxygen sensor made the exhaust hot enough to melt rubber exhaust mount under the floor board and rear seat. 4 cyl 2.0.

p.S. Car came with a jumper wire that ran from a electrial device on the drivers side of radiator to fuse box.
tried car with wire connected and not connected, with no change in black smoke. HELPPP. EXPECT $$$, BOB

would like specific instructions, on how to test componets. If possible

check engine light on, went to AUTO ZONE they said it is too old for their machine to test, motor is at least a quart low on new oil, removed plenty of gas and oil from engine and oil filter