Abstract: A passive type of filter apparatus for an automotive type cooling system utilizes a conventional gasoline fuel filter for filtering a portion of the liquid coolant moving through the system. Pickup and return conduits are positioned within the coolant flow circuit, and part of the coolant flow is diverted through the pickup conduit so as to pass through the fuel filter. After the coolant has passed through the fuel filter, it is returned to the coolant system through the return conduit. Two different types of sight glasses can be employed in the system to allow a viewing of the coolant flow and to also provide an indication of when the coolant is no longer acceptably clean.