so i have a 1995 del sol si. it has a B16 swap, and for some reason when it idles just under 1k rpms, the car vibrates in a pretty intense manner. I dont have this problem when the engine is cold, or when it idles above 1k rpms. Usually when its cold it will idle at 1500-2k rpms. And there is little to no vibration at all. Why does it do this?

 

Are you inquiring into the vibration issue or the idle issue?

 

the vibration issue

 

You said it's a swap, check your engine mounts.

 

Vibration issue could be misfire. Depending on the cause of the misfire, it could misfire only at low RPMs when warm or only at high RPM. When you get it idling, do you throw any codes? Flashing CEL? Does your CEL work?

I had a similar problem with my car because on of my plug wires was going bad.

 

I think misfire could happen at any rpm range, the higher the more noticeable if I'm not mistaken...

 

Yes, if it is misfiring at higher RPM it will often be more noticeable. However, many times the misfire is only present in a certain range. Even misfire at idle can cause extreme vibrations.

 

Mounts... maybe rear mount bolt is loose.

 

how would i know if it was mounts or misfire? are there any obvious signs that will show me which problem i have?

 

If your CEL is working, it will be flashing or at least be triggered and show misfire codes (at least this is so on ODBII, I'm guessing same holds true for ODBI) and you may hear strange popping sounds from the exhaust or smell fuel from the exhaust.

A bad mount should be fairly easy to diagnose. Jack up the front end and take a lood. I had a bad mount on my old car that caused a CLUNK whenever I hit the accelerator or let off of it. The rubber in the mount had deteriorated and allowed the engine to torque itself up and down, causing the mount bolt sleve to smash against the outer metal of the mount. But bad mounts can cause various different problems, depending on how bad the mount is and which mount is going. Visual inspection should be fine to determine the bad mount.

 

I'll go for mounts, I had an rx7 that had that same issue, It vibrated so bad at lower rpm, my engine mounts bushings were very deteriorated, I replaced the mounts and the vibration stopped, actually my delsol vibrates a little when I'm at a stop in 1st gear and engine is idling, maybe is time to replace them aswell.