What do you do if a lot of white smoke is coming out of the muffler?

Use fuel system cleaner, if it doesn't help you could have a problem with the cadalidic converter or the head gasket.

This could be because you have coolant burning inside the combustion chamber(s). If you are leaking coolant, but cannot find the leak i.e. no puddles, then it is probable that your head gasket is leaking, your head is cracked, or both. If you are handy with tools, rent/buy a compression tester. If you have low compression in two adjacent cylinders, you probably have a blown head gasket. Not an easy fix. Sorry to say that.

You may be able to get a solution from AAP that can fix the head gasket leaks. I do not know what the brand name is but they say it works. Just drain the coolant, rerun the solution through, flush and add fresh coolant. I have never tried that though.

Good luck.