OK First there is only one Coolant Expansion tank on a 2.3 190E. Second the Heater Valve is located on the passenger side under the cover that is under the wiper arm. Take that cover off to expose the fan enclosure and the heater valve is right there on the passenger side. Third if it is the core, you can check by taking the glove box out to see the one side and removing the lower panel on the drivers side and the knee bolster to see the other side of that. Look for coolant around the plastic endtanks and if you see it then it is the core. I had mine explode and burn the crap out of my ankles just about a month ago. Have to remove the dash to get it out and new one in. Took me 5.5 hours to do mine by myself and it was the first time I ever removed the dash. There is actually a thread at 190revolution.net's forum about how to remove the dash in a 190E.

Edit: No the heater valve should not leak into that drain. I hate to say it but it is most likely your core. Part was $90 online and as I stated above took 5.5 to R&R the core as well as check all vacuum connections in the dash and of course play around a bit in there.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....