Last night going down the freeway I did notice white smoke coming out of the tailpipe. I check the expansion tank for water when I got to where I was going, and the damn thing was empty. Is this a blown head gasket? The car runs good.

Given the LARGE amount of coolant you're losing, if it was an external leak I'd assume it would have shown up by now. I think you have to test, at least, for the possibility of a blown head gasket. I'd start with a simple compression test.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I'd check the water pump, mine did the same thing. For the longest time I had no idea where the leak was coming from, not even my local gas station could tell me, thats when I took it up to my local independent MB Shop. Cost me like $600-700, I forgot the exact amount. One question tho, Do you have any visual leaking, ie. puddles on the ground? Oh if you are leaking, and it's coming off the oil pan, thats prolly what it is.
Good Luck

I can't visually see where I'm leaking coolant, but it shows up right at the base of the firewall. Heater core???? None of the hoses shows any wetness on them. Had the front end up on jacks today, and after the engine got to operating temp I loosened the cap on the expansion tank 1/2 turn, coolant poured out of the spot I couldn't see at the firewall. The reason I wasn't noticing fluid on the ground before was that it was being caught by the belly pan. Would never have known that without jacking it up. Thanks for the insights.

Just like to thank everyone for all the replies about my coolant loss. Turns out it was the head gasket.