You don't even have to wait for the engine to get to operating temperature.

Just run the cold engine for a couple of minutes and then remove the coolant system cap. If there is exhaust leaking into the cooling system, there will be some pressure escaping when you loosen the cap.

Mine does this. Whenever I stop my SDL, I have to pop open the hood and relieve the pressure in the cooling system otherwise coolant will leak back into the piston because of the cooling system pressure. removing the cap by hand is no big deal at a temperatur of 80*C. The car will belch smoke until I rev it up to 2000 rpm for a bit and the cylinder starts firing again if I don't do this.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)