Where do you see the coolant leaking from? The tail pipe? Or is the coolant simply disapearing from the radiator? If you can positively identify coolant comming from the tail pipe then the problem is a blown head gasket. Do not mistake water as coolant. Water always drips from the tail pipe when the car is warming up. It is condensation caused by the hot exaust and the cold steel of the exaust syatem.
If you cannot positively find coolant in the oil or exaust pipe then you probably have a bad cam or valve spring as Benzmac first suggested.
1993 190E 2.3
2000 Toyota 4x4 Tundra