Ok, I bought this 90 zx na never drove it. It was not running when I got it, due to the lack of maintain, timing belt idler pulley went bad and it jumped a tooth and bent a valve, so I got another engine with less miles, I replaced the timing belt, so I know it's good,,i checked all the pulleys and they all felt good. This is an automatic car. So I installed the engine installed new plugs ,thermostat,etc. serviced the engine and trans....so here is my deal the engine runs smoth, but it has a vibration, at idle and on up thru the rpms in nutreul the engine is not missing,and idles fine. First thought was harmonic balancer, or something belt driven. So I removed belt by belt until they where all removed still has vibration. I changed out harmonic balancer, since I had an extra, but no change. Put all back togather. Next I loosened the engine mount nuts and jacked up the engine to reset and retightened nuts still no change...the engine mounts are good,not broken...the engine does sound a little funny up front around the timing belt cover, don't know if its the cause of the vibration or is the vibration cause of the sound. I am a mechanic and I'm stumped. If its a timing belt pulley that has gone bad idk, wouldn't think it would cause this type of vibration. Maybe the engine mounts are so old they are hard
And transmitting the vibration the the body, that's what
It feels like. As being a mechanic for a very long time I call this a engine grounding type feeling, like the engine is making contact some where rubbing against the body. Or exhaust binding.....which brings me back to a timing belt idler pulley or old engine mounts.....and then the little noise......any input would be great....oh this vibrates in or out of gear. Idle and on up in the rpms. Thanks Derek 90 300zx na auto 2+2

 

Ok, I found the problem, it was the trans mount. Removed cross member supported the trans, and started no more vibration.....