sounds like a blown head gasket to me. if it hasn't overheated too much, the block might be salvagable. but if it's overheated a few times there is a good chance the engine is warped.
might just be swap time
to find out if it can be fixed you need to pull the head off and clean off the gasket. then check for warpage on both the block and head. then go from there. if it's not warped too much the head can be milled and the block decked. but to be honest, i feel that is always too much money spent for a d15. much cheaper to just get a replacement engine.
gimmie a minute and i'll try and find the head gasket replacement helm's page to give you an idea what kind of work is required to fix a blown head gasket
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