- First, a word of caution: Be very careful about flushing the transmission of any focus with more than 100,000 miles on the clock. Though the transmission's original-equipment synthetic oil does help to prevent varnish, there's no guarantee it could happen. Flushing a transmission with excess varnish build-up will result in almost immediate failure, which will require a complete re-build to correct.
Before you flush your transmission fluid, first use a synthetic-compatible transmission flushing additive like LubeGuard and drive the vehicle for 10-15 miles. This and other detergents commonly available will dissolve the built-up varnish before flushing, which will reduce the overall cost of the procedure.
What You'll Need
- You're going to need a transmission filter kit, and at least 10 quarts of either Mobil 1 Synthetic Multi-Vehicle ATF (Automatic Transmission Fluid) or Castrol import Multi-Vehicle ATF. You could use a jack and jack-stands to raise the vehicle, but ramps are safer to use when the engine will be running while suspended. you'll also need two clean, one gallon milk jugs and possibly 3-5 feet of rubber transmission fluid lines and hose clamps.
- First, remove the transmission oil pan, and replace the filter as per your kit's instructions, and clean the junk off of your pan-mounted magnet. Refill the transmission with 3-4 quarts of fluid, and run the engine for five minutes. Disconnect the stock transmission cooler lines from your radiator, and have an assistant start the car while you watch the tubes. Have them shut the car down as soon as you identify the output line (the one that's squirting fluid) and the intake line (the one that isn't).
You may need to extend the transmission cooler lines a bit with the rubber tube mentioned earlier, but you'll need the lines to hang down almost to the ground. Put slide the output-line into an empty milk jug. Fill the other jug with fresh fluid, and drop the intake line inside. Run the engine until the empty jug is almost full and dump the used fluid into another container. Keep the fresh fluid topped off and the intake line submerged in fluid at all times.
Continue flushing until all of your new fluid is gone, and reconnect all the fittings. Back the car onto level ground, and check the fluid level while your car is running, topping off as need be.