I recently had a major internal coolant leak which dumped alot of antifreeze into the crankcase. Thinking I had a headgasket leak or internal crack I changed the oil twice and refilled the radiator. I drove around for about 30 mins., no overheating, no contaminanted oil, no drop in coolant level--with the heater turned off. As soon as I started using the heater the coolant finds its way into the crankcase again. I can't find a good diagram of the coolant flow to help isolate the problem. Is there a way for the coolant returning from the heater core to find its way into the crankcase only when there is flow through the heater? Any ideas would be appreciated before I have to drive all winter here in Ohio without heat or spend a weekend tearing down the engine looking.