Dear Tom and Ray:
Bonehead move on my part: While changing the oil for the first time in my 2005 Toyota Sienna van, I could not locate what looked like a normal oil drain plug. I ended up draining out the transmission fluid and then overfilling the motor oil. I did not realize my mistake until I was about two miles down the road, wondering about the rough gear shifting. I returned to my shop, added transmission fluid and drained/refilled all the motor oil. I drove the car four miles and up to 55 mph with no transmission fluid. There doesn't appear to be anything wrong with the transmission, as it now shifts smoothly. What damage could I have done to the transmission? What damage might I have done to the engine by overfilling the oil? — Tony
Tom: Congratulations, Tony. You are now eligible to join the Bonehead Support Group. Send your application to my brother. He's President for Life.
Ray: Gee, I'm surprised you even got the van to move at all if you really drained all of the transmission fluid. Normally, it'll just sit there and refuse to shift into any gear if the transmission is empty.
Tom: My brother would prefer you not ask him how he knows that!
Ray: So I'm guessing you didn't drain out all of it. After all, it's pretty near impossible to completely empty the transmission just by opening the drain plug. A good amount of fluid inevitably remains behind in the torque converter and the valve body. You obviously still had enough in there to move the car.
Tom: The answer to your question is: It's impossible to know exactly what damage you did. Obviously, you didn't do catastrophic damage, because the van appears to be running fine — at least from the point of view of a guy who doesn't know what an oil drain plug looks like. That's good news. What you don't know is what less-significant damage you did that will simply shorten the life of the transmission or engine.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)