[Carguy75]

I would not flush anything. You will risk dislodging sludge and other sediments in the engine or transmission and cause problems. I would flush only the transmission by manually flushing it with fresh fluid by opening the transmission cooler return pipe and pouring fresh fluid until the old fluid is removed. However, do not use a suction type of flush system or power flushing on your transmission. I would change the sparkplugs due to the age of the plugs. New plugs can not hurt the engine and maybe give you back some lost MPGs. To my understanding the transmission do have an mesh filter in the pan which can be cleaned. I would probably wait until the 200K mark, but it can not hurt to do it now if you want. If the engine is running fine, just keep doing what the previous owner did and leave well enough alone.

 

[Carguy75]

At 120K, don't flush it. And how would you drain 12 quarts? Even if you drop the pan, you won't get everything. The fluid in TQ convertor won't come out. If you use the method of pulling the cooler line, you still won't get it all. Doesn't really matter though, it will help.

The cooler line method will remove all the old fluid because the new fuild will flush out the all of the old transmission fluid in the system. It is like flushing out your old brake fluid in a way if you did that job before. However, with the transmission fluid you will add new fluid in to the transmission dipstick hole while the FX is running and the old fluid will be pushed through the transmission cooler outlet into a drain pan. You keep adding new fluid until only new reddish transmission fluid is coming out of the transmission cooler outlet instead of the old dark used transmission fluid and then you will stop the process. That method will remove 99.99% of the old transmission fluid.