Very rough

[RagingPanda]

I drove my 1990 S13 today and something weird happened, the car shook extremely hard. I don't know what happened because it was the first time ever when my engine shook so hard. It's like the car can't get something; as if it's choking. There also seem to be a lot of back pressure because when I let it idle popping sounds can be heard. I will do the Seafoam cleaning steps tomorrow with the oil change, so I'm hoping it'll be better after that.

[NISTECH]

maybe chain skipped time. maybe also the wiring harness along the valve cover has problems. try shifting it forward and back see if it effects the idle at all.

[RagingPanda]

I used some Seafoam today(oil, fuel tank, and brake booster) and the car runs really great now. I've also asked a mechanic that I know and he said it might be the fuel system. He just told me to try and pump 93 octane instead of 87.