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I recently did a timing belt/water pump change on my own. When I changed the water pump i drained the coolant, and according to my mechanic, I blew my radiator by not running the engine with the cap off after changing the coolant. According to the mechanic this lets air bubbles form in the radiator, which can cause pressure that damages the radiator.

Do I need to run the engine with the radiator cap off whenever I add coolant to the radiator or only when I drain/flush the system?

Also, can I take off the cap to the coolant overflow container and fill that without needing to run the engine with the cap off the radiator?

honda civic 2001

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    Make, model, year, engine size? What "major issue" And can you describe more completely, exactly what you did? – Move More Comments Link To Top Jan 1 '15 at 19:13
  • Generally you would run the engine only when you're draining/flushing the system because this will eliminate any air that still exists within it. Air is compressible and will turn to vapor once the engine becomes hot enough which is I believe what caused your damage. You can take the cap off the coolant overflow container without running the car; however, if the cap is a pressure cap do not take it off until the car has completely cooled. – gh0st Jan 3 '15 at 0:56
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Yes, run the engine

After you do any work involved with draining the coolant you will either have an exact process to remove air from the cooling system or you will simply run the engine.

If you don't the gasses (air) will expand at a greater rate than liquid when the engine get's hot. This pressure of expanding gasses can blow out hoses or weak radiators. Running the engine helps to get any air bubbles out of the system.

Do I need to run the engine with the radiator cap off whenever I add coolant to the radiator or only when I drain/flush the system?

Yes. You do it whenever you drain coolant out of the system unless otherwise stated by the manufacturer. On your model, you run the engine to get out the air bubbles from all of the various water galleys in your system.

Also, can I take off the cap to the coolant overflow container and fill that without needing to run the engine with the cap off the radiator?

Yes you can. You don't need to remove the radiator cap to fill the overflow. Just fill it to the appropriate marks on the side of the overflow container.

  • On that last part I think he's talking about adding fluid to the reservoir to top off the system, instead of topping off from the radiator cap. And not just filling the overflow. While it would technically work as long as the radiator cap vent valve is in good condition. It's not advisable. – Ben Apr 10 '16 at 3:35

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