1994 Subaru Liberty warm start

What color is the smoke?

If it is black, this would indicate that the fuel system is injecting too much fuel or that the throttle body is very dirty (which restricts airflow into the engine).

Try cleaning the throttle body, using carb/choke cleaner. Hopefully this will alleviate the problem and is less expensive than having the injectors cleaned.

Also, check the air filter. If it is severely clogged it could cause this condition (although that's along shot).