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.

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1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)