I’m a dumbass :(

Today I was replacing my radiator and I was draining all my coolant into my drain pan. Since I had recently done a flush my coolant was basically brand new, so I decided hey, why not just pour the fluid in the drain pan back into the new radiator and save some money on coolant since the stuff I drained is pretty much new?

Long story short, I forgot I still had a small amount (if i had to guess maybe 1/5th or 1/4th of a quart) of used automatic transmission fluid in the drain pan from when I drained and filled my transmission. I pouring in the old coolant into the radiator and noticed red streaks in the coolant but by the time I really noticed and put 2 and 2 together, almost all the fluid from the drain pan was in the radiator (I know I’m a dumbass)

Do I have to flush the radiator now? Or is the amount of atf so little that it wouldn’t hurt anything in the radiator? (Again probably about 1/4th of a quart max was put into the radiator)

If I were you, I would still wash the radiator. Even if it will be costly, you will be sure that the car will not break down and you will not have to pay more money for repairs.

I personally wouldn’t lose any sleep over it.


Why not just drain what’s in the radiator and refill with fresh coolant. Only takes a few minutes. If nothing else, it gives you piece of mind.

How much AT fluid dissolved in the antifreeze? Ethylene glycol or another, similar antifreeze chemical might dissolve all the AT fluid. Once it dissolves, it’s not likely to come out of solution. Drive on.

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ATF and motor oil in the coolant will severely soften rubber hoses eventually if not sooner. Dish washer detergents like Cascade will clean the oil out.

Did you start the car after adding the ATF? If not, just drain the radiator once more and add fresh coolant. No serious harm done.

I’ve got a 4" diameter rubber hose between the two pans on my custom transmission. It’s constantly holding ATF. It’s basically the same construction as a car coolant hose and 15 years old now… a pint or two even of ATF in gallons of coolant is unlikely to cause any issues IMO. YMMV.

I feel certain that the 4" hose connected to the transmission pan is rated for oil and not water and coolant @TwinTurbo. There are critical differences in the rubber carrying various liquids.

Well I built it and i fabricated all the custom parts and I bought the hose. But you know better…ok.

Anyway if you believe that tiny amount of oil is going to cause the destruction of the rubber in the cooling system, I’d call that alarmist.