first off i have a 1996 240sx. ive been driving it for a while and decided to wash out my engine bay. i put a bag over my distributor and the wires. drove it for about 20 min and my check engine light came on. i noticed when im at a light that my rpm's drop to about 500 and feel like my car is about to stall ??? when i bought my car the last month i replaced the o2 sensor cause the check engine light was on and that was the causing the problem. it went away for a while and now its back. could it be the knock sensor or what ????? help