I have a 98 Dodge Caravan that is leaking water but even though the radiator is empty or close to it the van does not over heat. What could cause this?

Do you have white smoke coming out the exhaust? If so you have a blown head gasket.

Agreed. Pull spark plugs and do a cooling system pressure test. This will cause it to leak without engine actually running. If coolant gets in the spark plug holes or you see it in your oil then blown head gasket.

No smoke or coolant in the oil.

Did you replace the radiator cap? I'm running out of idea's.

I had a simular issue that drove me nuts for some time and costs nothing to check out. Back behind the housing where the water pump bolts on there is a 2" piece of heater hose that could be dry rotted. You wouldn't see a leak until the coolant was hot and streaming. Take a look and if nothing else, cross it off the list.

Water outlet hose feeding the heater or the front/rear heater plumbing at the firewall passes over the exhaust crossover from the front and the exhaust manifold. A small leak that only occurs under pressure evaporates/boils without hitting the ground. Check the hose clamp at the heater outlet nipple. The engine isn’t overheating (yet) because you can't see very far down into the radiator. There’s still enough coolant to have enough effective radiator area to cool the engine as it’s loaded. Different loading or higher outdoor temperatures can tip the balance and the engine may overheat rapidly. A long uphill highway grade, towing, and higher highway speeds all require sustained higher engine output and heat production. Higher outdoor temperatures reduce the temperature difference between the engine coolant and the air and that slows the flow of heat. There’s no way to know how close the cooling system is to overheating because the engine thermostat limits coolant flow to maintain 195° F at the outlet to the radiator. The outlet tops out at 195° until there isn’t enough cooling and the thermostatic valve is fully open and the radiator isn;t shedding enough heat to prevent the engine temperature from climbing. At low outdoor temperatures the increasing coolant temperature only boosts the already large temperature difference and the heat flow by around 20%. At very high outdoor temperatures the heat flow is always lower but the percentage increase in the temperature difference is much greater. E.g. 195°- 100° = 95°, 235° - 100° = 135° ; 135/95 ~= 140% as it approaches boil-over a 40% boost.