when I drive down the hills on the way to/from LA (Grapevine), the engine temperature goes low, and it appears in the same level as the car just started in morning. It happens in the summer too. In the winter it gets so low that the transmission is disengaging the overdrive, and it takes some time until OD is available again. Usually it happens in the last downhill slop into the flat hwy 5, and when everyone drives 70mph around me, I am restricted to 50-55mph, because above that the engine start screaming for help. I noticed that some members are from southern CA, and I wonder if it is normal. about a year ago the coolant was changed with Toyota's 50/50 mix (according to the mechanic), even though I always had this problem. While going downhill, I let the engine do most of the braking. Could it be the thermostat? any other possibilities? I don't want to ask my mechanic, because he recognizes my limited knowledge, and volunteers to replace any part that I mention in my questions. Thanks for your comments suggestions.
BTW - thanks to all the members who responded to my previous post about the vibrations in high speed. It was the wheel bearings, as suggested by davidj92 and robmurr. Before I knew about this NG, I asked the mechanic if the vibrations are from the trans, he right away became certain about it, and told me that I better change it immediately unless I want to get stuck. Thanks to this NG, $185 solved the problem.