I learned more last night replacing my monovalve to fix my heat: This is exactly what happened when I spilled the coolant opening the monovalve. White smoke, burning off the exhaust. Mine went away after a minute, as it was only ther residue from what I spilled. Maybe one of those small coolant lines near the monovalve is drooling? Look on the firewall, just inside from the battery. You will see a switch mounted on a diagnostic port. Look under it. The little square guy with four screws and a round cylinder in the center is the monovalve. Above it is a hose, about 1/2 inch diameter with coolant. Look for wet around there, with the car running and warm.
Good luck, keep us posted.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)