91 Celica - WEIRD tranny issue!

Hello, new member here!

I have a 91 Celica that has a strange transmission problem. She hates the cold weather. LOL! No joke, she drives GREAT in the warm weather. But when its cold, she refuses to change into the highest gear until she feels like she's warmed up enough. But even then, she will randomly downshift again (all other things like speed, elevation, etc. constant.) When that happens, the RPMs (obviously) jump up. I always try to slow down when that happens, and eventually (5-7 minutes later) she shifts back into the appropriate gear.

I've owned it less than two years and have done A LOT of work on it. I am hoping to upgrade with a vehicle from a family member, and actually have the Celica for sale (and I hate selling cars......I don't think $1400 is too much for her!) Only 133,000 miles as she sat for a LONG time before I got her. I got all the kinks worked out except this transmission issue. Everyone I've talked to is stumped. And other than the "cold" issue, she shifts beautifully. I've been told to NOT flush or change the fluid at this point, so I haven't done that, but it's full.

I don't know if it's related, but the temperature gauge very rarely makes it off the 'C' mark. Sometimes when I'm idling it will climb to a whopping 15-20%, but generally it lives at or below the 'C.' I have never seen it at or above 50%.

Thanks in advance for any ideas on this issue!

Trans level checked properly? Follow instruction in book or on the stick with car level. Slow or no shifting into gear especially on cold days usually means the pressures are dropping and the fluid is shot and trans is way overdue for service with alot of wear. Is the fluid is stinky and dark? Some say not to change it, I'd say just drain and drop the pan then clean the screen. Replace pan gasket and refill it with fresh, proper fluid. Don't flush it. Ignoring trans service will lead to slipping and the only acceleration you'll get is a trip to the tranny shop for an expensive overhaul sooner.

Replace your thermostat. This may solve everything.

I've wondered if maybe the thermostat could be partly the blame since it doens't seem to be registering properly. But wouldn't that cause it to do this in the summer too?

My BF said I should just wait until the summertime to sell it and not say anything about the transmission (since it really does drive well, this is only a cold weather issue.) But I don't want to NOT mention it.....

Would dropping the pan and changing the fluid hurt the transmission at all? I"ve been told NOT to do a flush since it's an OLD car.....might damage seals and call for a rebuild (which I want to AVOID, especially since It's for sale.) But I've also been told that a car will get "used" to it's transmission fluid and changing it is bad too. Is that an old wives tale? I haven't checked it recently, but last time I checked it, it was reddish and I didn't notice an odor to it. Although I probably wouldn't know the difference.....LOL!

DO NOT FLUSH IT. But if you're skeptical about giving the tranny minimal service and it's only a "cold weather issue" for you then just let it warm up for 5 full minutes in the AM before driving it.