C6 slips on shifts when cold

My C6 is slipping on shifts when cold. Say on the cold mornings now usually around 30*-40* I start and let it idle anywhere from 1-3 minutes then drive off.It will slip on shifts, particularly the 2-3 shift, and worse when coming to a stop then accelerating again like for a stop sign. Feels like it's sort of between gears. If I shift it manually until I've driven a couple miles it does not slip at all. So clearly it's being lazy changing gears until things are warm. Truck is an '86 with allegedly 90k miles or so but has a reman trans and also a large cooler in front of the rad in addition to the in rad cooler.

What might this be? Old hard seals leaking until they warm up and get pliable enough to seal pressure? Vacuum/modulator/regulator on pump issues? Vacuum is pretty rock solid at 24" give or take an inch. Fluid is right about the full mark when idling hot, maybe a little dark and slightly bad smelling but it's not real dark and nasty cooked smelling, seems more like old and a bit overdue for a change.

Anything I can check or try free or cheap to resolve this or is it just a sign it's due for a rebuild? Plan is for a ZF5 swap but I'm not having much luck finding a decent used ZF5 within a few hundred miles. May end up having to bite the bullet and order a rebuilt unit since the few that do pop up people want at least half what a rebuilder jobbie cost but for a 200k+ mile piece that's cracked and welded all over. Would be nice to have the luxury of choosing when to do the swap rather than the C6 crap out in the dead of winter and need to do it then.

Too large of a transmission cooler could be keeping your fluid far too cold in the winter, causing you some issues. The C6 is regulated entirely by pressure in the valve body, so with the pump trying to push thick, cold fluid it may not be getting adequate flow/pressure.

Try bypassing the cooler (with a male/male hose barb and a couple clamps) and double check the fluid level.

Some coolers have a thermal valve that recirculates fluid back to the transmission, but I've had bad luck with these trapping too much fluid and starving the pump on my AOD.

I cured my old c6 problems several times by just changing the oil. I bet that will cure your problems. Turn the engine around by hand with the inspection cover off to find the torque converter drain and drain that, and pull the pan down and drain that. Then get ready, it takes about 13 quarts to fill it back up, but I bet it cures all your problems.

I did the same thing as you and cooked the oil a little bit, had a few shifting problems, changed the oil, and they went away. I never did rebuild that tranny.

I had same problem I ended up doing same thing as mentioned I changed fluid. I pulled the valve body and installed another used one which was worse than what I had as it turns out. so I swapped in my old one again because at least then I could get to work but because I had totally flushed the fluid out when I swapped it a second time the trans started working beautifully. so I would give it a shot. or just pull your valve body and rebuild it with new parts. after getting into another trans for the very first time (son and rebuilt one as a spare) all decision making inside the trans is done by the valve body so I would suspect (im no trans mechanic just a maint guy on production equipment) that most problems with lazy shifts and lazy to engage can be solved by a valve body. your maybe some thing much more I don't know but I n
know mine was taken care of by really nothing more than a complete fluid flush

In the trans trade, they call that "Morning Sickness." 80% of the time, it's the first sign of a worn out trans. Sometimes it's worn clutch plates but often it's low mainline pressure because hardened internal seals are allowing pressure leaks and they are harder and leak more when cold than after they warm up, plus the fluid is thicker and mainline pressure tries to be higher but blows past the seals. A fluid change can sometimes cure this, and you should add a conditioner with seal swelling agents. I'm not current on the best products. Trans-X used to work but Auto RX has one too. The main thing to remember about these kinds of products is that the cheap ones generally don't work because they only contain minute amounts of the active ingredients (esters).

Be sure to change the filter screen because a plugged screen can cause low pressure (it's not common, unless the trans is already shedding junk in large amounts... which means it's a zombie, read below)

Do something now! A slipping trans is killing itself!

Finally, if you do a fluid change, be prepared for the possibility of a negative result. When you drop the pan, if you see a lot of junk in the pan... metal and brown stuff... that's a bad sign of a possible zombie trans. When you put it back together with new fluid, there is the possibility of it slipping worse after. It won't be the new fluid killing it but more the old fluid was keeping it going. There are friction modifiers (FMs) in ATF that allow the trans to shift smoothly (essentially they allow the clutches to slip a little upon shifting). They get burned out of the fluid over time and when the trans slips a lot. So old, worn clutch plates and bands that would slip with the normal amount of friction modifier present, have a little grip without them. So, you put new fluid in with loads of FMs... slippage. Old fluid, no slippage. BUT, old fluid that's loaded with debris and worn out in other ways is also killing the trans and leads to a "danged-if-you-do-danged-if-you-don't scenario."

I'm not recommending this, but I remember a guy who had a zombie trans (and he knew it) but he had to keep it going. The old fluid was really bad and that was killing the trans too, but he knew if he put in new Dex III it would die quicker. He put in Type F, which has almost no FM, along with an aftermarket seal swelling agent. After, the trans operated almost normally for a good year, when he finally had it rebuilt. I have put type F into a trans that didn't need it and if you wonder what hard shifts feel like, try that and you will know. Racers used to do that with automatics to make them shift harder for racing (maybe they still do) but it will definitely be a life-shortener day to day.