fenderfatstrat6

****** I know this is long but i felt i needed to be very thorough*** please help Hey friends I am completely at a loss been messing with my 1991 240sx ka24de since the first day it was salt free and warm.... ok here we go ...... I started it up any time the snow would melt over the winter started like a champ every time... i take it out on her spring maiden voyage and it was good for about 5 miles then suddenly it starts idling really rough like so hard if i said ahhhhhhhhh loud in the car it would sound like i was bouncing up and down. so i say well its got to to be it needs a tune up or just to be driven a bit b/c it wasn't run hard over the winter just idled and reved for a bit . so i drove it ten miles to my buddy's place and it surly didn't feel like it had the power it once did. he thought it might be the injectors so we took them out cleaned them off popped them back in and the idle got violently rough and it wouldn't want to stay running unless i kept on it. so i had no choice but to drive it home pedal all the way to the floor and i was barely getting 80 out of her. it used 1/4 of a tank of gas for 10 miles back home WTF... next i think its the injectors so i go get 2 complete sets from the junk yard doesn't change anything, then i do plugs wires o2 censor cap and rotor still nothing changes. i was told by a 240 guy to check the oil it might have gas in it if i used that much gas. sure enough i changed my oil i kid you not 3 gallons of fluid gas/oil came out. yadda yadda i pull ecu read the code its reading knock sensor so i go get one from the junkyard, nothing changes, try another ecu nothing.. i tried to look over the vacuum lines the best i could didn't appear to be any issue with them/ also did the carb cleaner test with them. im about to give in to a raping at my local garage to get this thing fixed so as a last plea for help before my wallet takes a beatin' i decided to see what you guys have to say.
I would really appreciate any educated input you can give me !