Automatic Transmission Fluid in Radiator

I did something that is not to smart and added about 7 oz. of GM Dextron ATF to the cooling system expansion tank. The car is a BMW 3-Series. I am having the cooling system flushed. We probably drove less than 25 miles with the ATF in the coolant.

Question: Should I be concerned about severe damage to any of the seals, gaskets or water pump? Will a normal system flush remove all the ATF from the system?

Thank you for any advice!


You’ll be fine. The amount that actually got mixed in with the coolant will be miniscule.

Sleep soundly. You’ve caught your error early and done the right thing.

Thank you for your advice.


ATF is lighter than coolant so it will float. Since you put it into the expansion tank, there’s a good chance that none got into the rest of the cooling system. Cleaning out the expansion tank may be all you need. Be sure if you get the system flushed, they clean out this tank too, that may not be part of a normal flush.