I did a motor swap on an 02. Mind you I've done about ten in the last two years. Anyway after a bunch of fuel pump, immobilizer and other b.s. I finally got it started. However when the computer idles the car down, the motor shakes violently. What is that?

 

Lots of things. I would first check to see if all the cylinders are firing, getting their spark and fuel injectors.

Was the engine disassembled/rebuilt and then installed? Maybe the timing chain on the cams is one tooth off.

 

No it wasn't taken apart, as far as I know. But I'm not getting P0300, P0301, P0302, P0303, P0304, P0305, or P0306 codes.

 

While not having any codes is interesting, maybe this is an indication of a wire harness connector that is not fully seated.

try removing the wires to the ignition coils one by one and see if that make a change in the idling. Pulling a wire and the idle stays the same means that that cylinder is not firing. If the idle gets worse, then that cylinder is working OK.

Then check the fuel injectors. Use a stethoscope or something and touch it to each fuel injector. You should hear a clicking sound. No sound means that that fuel injector is not working. Compare how loud the sound of each injector with the other injectors to see if there is a weak one. Also, measure the resistance of the fuel injector coils. They should be about 15 ohms.