How to Clean Out a Coolant Expansion Tankby Richard Rowe
Clear plastic expansion tanks must have seemed like a good idea at the time, and they are in a sense. A plastic expansion tank allows you to see a fluid level, which makes keeping an eye on things much easier. However, those pretty white tanks also have a habit of trapping rust and mineral particles floating in the coolant stream and presenting them as an ugly orange stain. Restoring an expansion tank, however, is no more difficult than cleaning out anything else with a nasty, rusty ring.
Unbolt the coolant expansion tank, unplug the rubber hose and remove the tank from the car. This step may or may not be necessary, but it'll keep all those nasty chemicals from spilling all over the engine.
Fill the tank to the brim with undiluted calcium-lime-rust remover. You've got a couple of options here. The CLR remover will work best when hot, but don't heat the stuff up in your home, or you'll risk an inhalation hazard from the fumes. You could also completely immerse the tank in fill a 5-gallon bucket of CLR to de-rust the outside.
Screw the lid back on and allow the expansion tank to sit for at least an hour, preferably overnight. Swirl the tank around a few times to agitate the mixture, which will help release any clinging rust particles. After you've allowed it to sit, pour the mixture out and thoroughly rinse the tank out with water. Do not use any kind of soap.
Refill the tank with a 75/25 mixture of water and bleach, and allow it to sit for 1 to 3 hours. Most expansion tanks are made of polyethylene plastic, a substance often used to build chemical storage tanks. The tank should resist bleaching without any problem. The bleach should kill whatever rust is left in the tank.
Dispose of the bleach, wash the tank out with water and refill it with pure water. Allow it to sit for at least 30 minutes, and repeat twice more. The tank will still smell a bit of bleach, but it won't be enough to have an impact on the cooling system. Reinstall the now cleaned-up tank.