Toyota Repair: 87 Corolla FX-16 shifting problem, temperature guage, temp guage

Question
My FX-16 has an auto transmission with overdrive and will not shift into overdrive until the temperature guage reaches a certain mark.  If after it heats up enough and I get back on the road the temp guage begins to go back toward cold and engine shifts up to 3rd gear.  On cold days, 30's to 50's, the temp guage stays almost allthe way over on cold.  Could a thermostat change solve my shifting problem?  Thanks for the help.

Answer
It will lock out mid-high rpm preofrmance by dropping ignition timing, and lock-out overdrive until the ECU reads the coolant as warm.

And you're right on the money. Flush the coolant, change ther thermostat (the most likely problem), and change any pressure caps on the system as maintenance.
You really need to flush the coolant every other year & change the thermostat & caps every other flush (4 years). It's just good maintenance.




And for god's sake change/flush the transmission fluid no less than every 3rd year. >90% of transmission failures are caused by simple fluid problems. Mostly, old worn out fluid & low fluid!