The filter was done a year and a half ago, as well as the regulator and pump, so I am almost certain it isn't fuel delivery. Besides, constant loss of water would indicate a blown head gasket, right? Additionally, I would hear noise coming from the cooling system inside the car that sounds like bubbles in the line (I suppose through the heater core). Does anyone know how tough a rebuild would be?

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89 300CE
91 Supercharged Miata
99 E320 Wagon