Our 1995 850 turbo sedan has been a great car for the last 17.5 years and 200+k miles. Today, with temperatures in the mid-80s, after 2 hours of hard driving in heavy Chicago traffic (because we wanted to be at certain places within a specified period of time), the car would upshift up to the 3rd gear, but not into the 4th. After it was parked with the engine off for about 15 minutes, everything turned out to be fine again. Has anybody else had such an experience before? What would such a behavior suggest? Otherwise, everything in the car looks normal.

There is an ATF temperature sensor. If the ATF gets too hot, the tranny will remain in 3rd gear to help push the fluid through the radiator. The pump is dependent on engine rpms.

I suspect that you had a full car, had the AC on, and there was a lot of traffic.

Later this spring, when you get your garden hose out, you might want to remove the engine fan and take the hose and reverse flush the outside of the radiator to clean the dirt out of the fins. It might help. Just don't spray water on top of the engine where it will fill up the spark plug holes.

Klaus
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There is no present time, just the past and future

Klaus' response seems to be consistent with what I experienced. Than you very much Klaus.

Klaus' response seems to makes sense in this case. Thanks Klaus.

Did you check the level of the tranny fluid?
What color is it? and when was it flushed last?

Tranny fluid level is fine. Its color is fine. Last time the tranny fluid was changed was about 27k miles ago and ~4 years ago (I do not drive it as much lately, because I bought another car too). Also, no check engine light/messages.