91 240sx missing; all spark related parts check out, seems fuel related???

[TK240sx]

my 91 240sx is missing on the fourth cylinder, (pretty sure) plugs, wires and distributor rotor are good. Compression seems good. I could disconnect the 4th injector and it made no difference in the way it ran. I replaced the injector and it ran great until the next day when I realized my gas cap was still off. When I put the cap back on, it began again.Now it doesn't matter if the cap is on or off, and it blows white smoke and eats through gas like there's no tomorrow. Exhaust is very rich. I replaced the injector again in case the new one was faulty, but to no avail. I'm only driving about 12-15 miles a day and I eat about 1 1/2 gallons of gas a day and I'm bouncing all over the place with this miss. I'm lost, please help.