i bought a 1997 nissan altima with 115k miles. when i bought it was smoking white and plugs 2 and 3 werent sparking. guy told me smoke was from sitting for a while. put new ngk copper plugs in it and drove it home. about 15 miles or so. later that day i went to go to the store and everytime i put it in reverse it would cut off. so i turned up the throttle cable and the motor started shaking like crazy. when in reverse or in drive and i hold the brake down and press the gas to the floor it only goes to 1500 rpm. i check the check engine light it says misfire and knock sensor. so i bought a new knock senor and replaced it and it still runs like crap. i did a comperssion check from left 2 right 175psi, 90psi, 85psi, 150psi. does this mean i got a blown head gasket? car doesnt smoke anymore and it not overheating. any input would help. thanks for reading.