How do you get rid of rust in a car radiator?

Answer 1

Like all things, it depends on how well you want it to work.

Best, take the radiator out and have it rodded at a professional shop where the tanks will we removed, then entire radiator boiled in a caustic acid to take out the rust and deposits, rods are run thought the channels and all holes are repaired. Replace the hoses, clamps, thermostat and button it up checking for leaks.

Good is to drain all the fluid, including make sure the defroster valves are open to drain that fluid as well, take out the thermostat so water will run free though the engine, and push water through it until it runs clear. Then put in a high detergent solvent to finish cleaning out the interior channels, run the engine for a min of 20 min and perhaps much longer if it needs it. Flush out the radiator again when cool, until it flushes clear. Button it up replacing anything that leaks.

If the car is a junker, you don't intend to keep it running but just need to get it running for some specific short-term tasks. Empty the radiator, and fill it with clean water with an active flushing agent. Button it up and watch the temperature gauge. Use the engine checking for leaks and listening for the water pump to protest with a squeal. Flush it out when you can.

Water works better at heat transfer than anti-freeze (ethylene glycol), but it doesn't have the lubricants the water pump needs internally to keep spinning. Water alone long term will seize the water pump, and lead to rusting in deep cavities in an cast iron block.