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Q: Radiator losing water (no leaking) but overflow keeps gaining water

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I have 2007 Nissan navara d40 2.5ltr turbo diesel. I have noticed that after driving and the car has cooled down that the water in the overflow has increased. When I open the radiator cap you can hear air/pressure noise and the radiator takes about 3/4 to 1ltr water to top back up. Also when I run the car with cap off and rev it up it bubbles a bit and pushes some water out of radiator. I can't see any leaks and it's not running hotter than normal. Someone said that was normal due to an air block or something and because the two tanks are at different height

My car has 255000 miles.
My car has an automatic transmission.

Based on the information you provided, It sounds like you may have a blown head gasket. What happens is that the high pressure in the combustion chamber squeezes its way through a break in the head gasket into the coolant passages. The result is pressure build up within the cooling system that could push coolant out into the overflow tank. Be sure to check your oil to see if coolant has contaminated it. If so, it will look milky.

I suggest getting help from a certified technician, such as one from YourMechanic, who can diagnose the possible leak and perform a quick test to determine if combustion gasses are getting into the coolant. Best of luck.

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