during hot weather (+80F), and city driving conditions with aircon on, the temp gauge rises to between half way and max. on switching off aircon, gauge drops to halfway. with highway driving temp gauge is between halfway and min even with aircon on.
service personnel looked at problem. both fans working ok. belts ok. coolant flow ok. they suggested thermostat needed replacing. this was done. problem still exists!
should coolant reservoir be filled to max level with hot engine?
if so this may be the cause as in our case it isn't.
maybe we have coolant leak.

 

Id get a compression test on your radiator and check if it leaks. Your radiator should be full. Have you checked your overflow tank to see if there is any coolant in there??

 

test radiator cap, also when it overheats pull over and chk the fans make sure they work properly you could have an intermmittent working fan or 1 that has too much resistance thus not spinning fast enough

 

If you do the pressure-test & can't find any leaks, you can check out the radiator itself. Maybe it's got crud built up inside, & needs to be cleaned out. Try the flush/cleaning stuff you can put in there, then clean it out & get fresh anti-freeze. Scale & crud builds up & interferes with cooling. Sometimes it's bad enough to need a new radiator.

After replacing thermostat, I hope they got all the air out from your cooling system... You should see the reservoir level get higher when it's hot, then it gets sucked back into the radiator when it cools down. Don't let it get EMPTY.