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Inline Heater Hose Coolant Filter. It also regulates the pressure entering the heater core. The 5/8” size is the most popular. Note: May not work with dual heater system as in SUV or Van. Patented Filter: Never has there been a filter that not only looks great, but lets you see dirt and debris...
last week someone posted a pic of an engine block with the cylinder head removed, the walls of the waterways where covered in the light brown crud which blocks the heater core.
apparently it is sand left behind from the casting of the engine block, i'm not conviced, almost looks like its caused by a reaction between the antifreeze and the cast iron,