I have this 1.7L civic that overheats a bit when down shifting. Temp gauge goes up and coolant overflows the reservoir. This only happens when slowing down from 5th gear. Let me know what y’all think. Appreciate it.
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I’m thinking you’re going to need a head gasket soon!
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+1 to @PvtPublic , Honda engines of this vintage have a history of head gasket problems. The engine is otherwise solid. If caught and fixed early the engine will continue to provide good service. If the rest of the car is in good shape this may be a good investment to keep it running.
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Check to see if the cooling fan is running backward. I kno that this is a Zebra to the headgasket’s horse but I have run into it twice. On both vehicles they would run fine at highway speed because the air coming in would overwhelm the fan, at low seeds the car would run fine because the fan would cool the engine by pushing air forward through the radiator but at 45-50 mph, there was no air movement through the radiator and it would boil.
I suspect the overheating is happening while downshifting has nothing to do with it.
Overheating can be caused by a faulty thermostat, fan operation not good, clogged radiator, or even a partially clogged catalytic converter. Coolant overflow can be due to a weak pressure cap also.
Checking fan operation for the proper blade rotation as mentioned is also a good idea. Some years ago I had a SAAB that was overheating a bit but not severely. The cause was one of the 2 fans had a reverse rotation of the blade. That in turn was caused by the 2 wires in the electrical connector being reversed. Unlocking the terminals and swapping them fixed that issue.