Is importing a Japanese car to the us illegal?

If the car is over 25 years old, then I think you can import it.

For newer cars:

If the car is the same as a model that is sold in the US and you can get a letter from the manufacturer stating that the Japanese version is basically the same as the US version (i.e., it meets all the US safety and pollution standards), then you should be able to import it after working through a bunch of red tape.

If the same model is not sold in the US, it is still technically legal to import one but it is not practical. To import it, you must modify it and show that it meets US pollution standards and I think some safety standards. You can find a company to make the changes, but it is VERY expensive and takes a long time to do.