Automatic tranny 5sfe '98 Camry just reached 260 000kms still going awesome very smooth still plenty of power in there but I noticed something today that bothers me pretty much.

Don't go DUH (at least not yet), here's the problem : I'm going up a pretty steep hill and the red light at the top turns red so I stop, sitting there nose pointing uphill. When the light turns green I let off the brakes and something this car has never really done before : started rolling backwards slowly. This car has never done this before. And I go up this hill every day of the week.

Is my tranny starting to slip or something, it shifts a bit rough sometimes especially second gear but when I give more gas the transition is smoother, I'm taking it to a tranny shop next week for inspection and flush anyway but still I'm wondering.

One thing : It's a serious snowstorm outside right now, but I know it wasn't the tires slipping. I'll test it out on dry asphalt tomorrow but I'm still wondering if there's a problem what it might be. Maybe the weather influenced some other component!? I dunno...

When's the last time you had the transmission serviced? If it's been a while, change the fluid in the transmission and the differential.

You've got a 4 cylinder & automatic transmisison have a torque convertor...
There is no direct coupling betwene the engine & transmission. it takes a big load to reverse the direction of the input & output shafts... But it's not something that can't happen when you ahve a weak motor idling with a big load.

Could it be possible that the car idles too slowly to maintain a certain drive on the transaxle causing it to glide slowly backards when stopping uphill and not holding the brakes? The car idles at 300 rpm when it's hot.

I cheked on the EGR valve and modulator today but even though the car doesn't seem to want to stall or anything, they are FILTHY. Thoroughly rusted and really, really ugly. I don't think taking them out for a cleanup will do anything since they look so worn but I'm thinking about getting them from a scrapyard and swapping them...

300 rpm is indeed a little too low, though I've seen a 12R engine idle at what looked like 200 rpm before. (Toyota engines are known for their very stable idle characteristics.)

But yes, 300 rpm will not produce enough hydraulic friction to keep the car from rolling back. Even a normal idle speed of 700 or so will still be unable to keep a car from rolling back if the gradient is steep enough.