1996 F150 5.0 automatic transmission. 298,000 miles, I've had the truck for years. Engine is running strong, past few days when I accelerate from a full stop the transmission was slip and take one or two seconds before the vehicle starts to move. Began last week and I've noticed it only seems to happen in the morning when the engine is cold. I live in Southwest Florida so it's never below 50 something degrees when I'm going to work in the morning even now in January, so the engine is never in freezing temperatures. As soon as I drive a few miles and it warms up the problem goes away. I'm definitely not a transmission guy and definitely wouldn't even know where to begin diagnosing the problem.

My rear differential failed about 2 years ago and I got a whole new rear axle from a salvage yard differential and all, but have had zero issues with it since.

Just over a year ago my transfer case failed and I got a transfer case sent in from a salvage yard in Pennsylvania, was very hard to find a Borg Werner in the correct model. Didn't do that myself had it done by a professional mechanic and have had no issues since.

Feels like a transmission issue and not the differential or transfer case but thought I should at least mention those things. No transmission fluid is leaking out, I thoroughly checked for that.

For now the truck is definitely drivable, still my daily driver, but I'd like to try and get to the bottom of what's going on, any suggestions or advice is much appreciated.

 

Most likely the hydraulic seals inside the tranny's valve body have hardened some allowing fluid bypassing when cold.The clutches can be affected as well. The condition (morning sickness) usually worsenes during the winter months.As the fluid warms up, the seals soften some and begin to function as they should. It's all caused by old age and to some extent the acidity content with old worn out fluid.You can either warm up your truck prior to driving, rebuid your tranny or try a chemical additive as a last resort which may have a seal conditioner in it. The additive can make things worse sometimes. I would atleast try dropping the pan, cleaning the pan and magnet and refilling with Mercon V. fluid. Looking in the crystal ball,I see a new or rebuit tranny in your future.

 

True story, if it is full of fluid the lip seals are hard
Trans medic is one of the additives that helps
We used to joke amongst ourselves "put a can of this in there and overhaul the trans next week"
Some of them it actually works on

 

Is it possible to replace the seals without rebuilding the whole transmission? Not done by me of course, way beyond my skill set, Id have to find a transmission shop

 

No
Not really
By the time someone pulls it all apart to replace the seals, you will be getting new clutch packs and the rest of the rebuild at the same time

 

300K miles and not a mention of getting a trans flush & fill. I can't possibly imagine what might have happened. LOL.

 

I was always told transmission flush would do more harm than good so I avoided it. Definitely not because of me being cheap. Bought the truck with 146k miles and to my knowledge it had never been done. Everyone I talked to including my mechanic at the time said a flush would do more harm than good so I just left it alone. Plus back around 2014 or 2015 a good friend of mine in a 2001 Jeep Grand Cherokee had a transmission flush and within a month his transmission failed and he needed a complete rebuild.

 

trans FLUSHING is not good practice , trans service , which is in your owners manual by the way,... IS. you drain whats present, you fill it with new stuff. the biggest thing is that there is fluid additives and properties that break down, and replenishingthem while also ensuring the filter/magnet have capacity to catch abrasive material wear.
If you dont do that, then they overwhelm and your original conventional ATF is bad before it was even 2000

and its 2025 now.