I have a 2002 Gt with Automatic and what i have found out is they need to have the fluid changed at regular intervals if you expect them to perform well and stay working. I got the car with 65K on it and all the performance gear was already installed, got no receipts for anything and no list of parts. seller didn't advertise anything special, Saw it had tons of extras even seat upgrades. Got it for a steal, at 90K I noticed some weird things like strange shifting into 2nd while driving normal, like a chattering of groaning sound. i immediately changed the fluid, it was blackish brown. Never was changed since new or since the trans was modified. I bought 14 or more quarts of the only available Mercon V fluid I could find, it wouldn't be a bad idea to get it directly from a Ford dealership. Do not use SYNTHETIC, I read up on these transmissions and there were only a few made to run synthetic I think they were both vans. Using the wrong fluid will destroy them, quickly.
I did not do a complete change even though the fluid looked terrible, I'm not sure if it was the proper way to do it or not but I have a FORD computer program that is a dealers service program from FORD that specifies to not change out all fluid. They have a special machine that they use if you want to change the entire system and if you don't have that machine you may create air pockets that will not allow fluid to pass through properly. so i removed the pan and changed the filter and drained all fluid that would freely drain out but i didn't remove the torque converter plug. I figured that may be a bigger job and it was not stated to do so in the instructions. I still used over 10 quarts of fluid when i refilled it.
My shifting problems went away instantly, if I take off too fast and break my tires loose my car will shift into 2nd gear while I'm burnin both tires and continue to do so until it tries 3rd gear, at that point it stops and downshifts back into 2nd again. When the tires are gripping the pavement and I'm driving it hard either shifting up from 1st or down shifting it will leave rubber on the pavement. I did another change again after 15K, and again, I just drained it and refilled, this time it was still red and pretty clear. Never had another problem and I still have never missed a shift, something I cannot say about my old '89 Gt with a 5 speed.
The only issue I have (at 125k of hard driving) is after I drive in high heat (Phoenix, AZ over 115*F in the shade) for an extended amount of time (freeway for 1 or 2 hours), I will get some fluid leaking out the rear seal. It only takes a drop to create a cloud of smoke, also if it's even slightly over filled it will come out the top of the check/fill hole, I know they don't have a check/fill on the newer models so it's probably even more critical to fill them properly.
I have more fun driving this automatic than the 5 speed, once you get used to how it works and when it shifts you will enjoy not having to push a clutch in, especially on your way home in rush hour traffic and even more so if you have a beefed up clutch installed.
