smoke from tailpipe

Had problem with smoke coming from tailpipe, rebuilt heads nothing changed. Motor ran great for about three days, then started smoking again. Got motor from wreaking yard, installed yard motor good for 90 days, ran for five hour to test setup yup, started smoking. I changed 02-sensor no change. motor idels fine. whats up? Forgot vehicle is 1995 Plymouth voyager LE 3.8 engine.

What color is the smoke? If it is blue you may have a problem with the rings. If white you may have a bad head gasket or intake manifold gasket. If it is black the cat could be bad or the mixture may be too rich. Remember there should be some moisture/smoke coming from the pipe even when everything is normal.

thanks but when I changed engines problem still happens, Used same intake checking into that at present. Could be some sort of sensor but what one?

I doubt that it is a sensor problem other than the O2 sensor causing the engine to run too rich. You may have a bad intake manifold gasket causing water to enter one of the combustion chambers. If you let me know what color the smoke is that you see, I may have more ideas on what is wrong.