What engine is it? it might be the hose coming from the firewall out of the heater core into the engine block. typically you can see the coolant spewing out once the pressure builds up, or you can go out and feel for wetness while the engine is cold. the hole might be near the hose clamp, but just go look around the area for signs of leaking,. if you don't see anything, turn the car on and let it get up to operating temp where there is pressure in the system then go look at the hose.

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1990 190E 3.0L