Forgive me if this has been answered, I have searched but could not find this scenario. I recently purchased a 2000 F150 5.4 4x4 4r70 with 328,000 on it. Crazy right but it was the original owner, well maintained, I had a mechanic with Car Chek go through it and perform a complete pre-purchase inspection etc. All seemed good. Now 500ish miles later I notice when it is fully warmed up, (45 minutes of freeway) and I coast for a while (downhill) when I gently get back on the gas the engine will rev to level and start pulling, then after half a second fall by 2-300 rpm. It is possible it always did this and I just didn't notice but since I'm looking for it now, I really notice it. It doesn't happen every time but probably more often than not. Perhaps the torque converter unlocks during coasting and then locks back up under load and this is normal? Or perhaps this transmission is on it's way out and that's why Grampa sold it. Perhaps he changed the fluid before selling it and now the problems are setting in? The fluid looks good, does not smell burnt and is at the correct level. There are no DTCs. It shifts up through the gears with no issues and seems work just fine other than this issue. I always drive stick so perhaps I'm just not used to this. Any ideas, advice or help would be greatly appreciated. Thank you for reading this.

 

They all do that.

That's exactly what's happening, and it happens because that was the way it was designed.

Changing the fluid in an old transmission WILL NOT cause problems. It won't fix a broken trans, but it will not cause a good trans to fail. That's an old myth that just won't die.

Just keep driving it. It's working as designed.

 

Thank you. That puts my mind at ease. I still want to change out all the fluid and filter so I will KNOW what's in there and to put my mind to ease even more. Do you recommend any particular brand of Mercon V? I ordered 2x 5 quart of Motorcraft from Amazon but now realize I will need more. There doesn't seem to be much on the store shelves around here. Would mixing Valvoline, Castrol or even SuperTech with the Motorcraft matter as long as they all say Mercon V?

 

It might not. I've never tried mixing fluids. I'm not brave enough to do that.

I often recommend Mobil 1 Synthetic ATF. It is MERCON V rated. Any fluid that has a MERCON V rating will work fine.