I took a very long trip yesterday with a 1990 celica (4afe 1.6L 5spd) which I have owned for a few years. I guess the roads were more hilly than I could feel usually, because in overdrive going 60mph (~2.8k RPM) I got a CEL. After downshifting to 4th, a few seconds later the CEL went away. This happened about 7 times during the whole drive. No actual symptoms were noticed besides CEL, but it was of course worrysome. Also, because all of the times it happened were der to increasing elevation it could have something to do with mixture, but I am not sure if this is common. All maint items were up to date. This is an OBDI car, which I am not too familiar with - i don't have a specific scan tool for it but I am not sure if it would work anyway because the light keeps going back off again. Any idea what might have triggered the CEL that I should check for? Any way to be sure I am not putting too much stress on the car when going uphill in overdrive?
Checking codes is not hard for a diyer, and should be checked in case it does give you some information. Here is a resource that shows the list of codes and how to manually retrieve them.
Retrieve the codes first, as that might point to a problematic system that should be diagnosed before asking the question of whether or not you are putting too much stress on the vehicle going uphill in overdrive.