Erratic temperature Gauge

Check fans & coolant temp sensor. With a/c on the fan/fans run but this switch to run overrides the computer to turn fans on if fans are good and all systems in working order. With heater on this does not occur. A sensor or relay may be failing but still works most of the time. Coolant temp or and Fan relay(s). I would also make sure the cooling system has been bleed of air, Fans are not getting hot then not working full speed or very little, which is one way electric motors go out. Try after car is up to temperature while fans are running with a/c on and a/c off to see if both (if equipped or the fan/fans) are working when turned to off or turn on and importantly too is while at temperature while fan should be on, with a/c off see if fan(s) run with and without heater on. After car is at temp also test the fan relays and test the coolant temp sensor/switch. With this issue not a bad idea to just put new oem one in to be sure, as with continued over-heating problems the chance of a failed head gasket rises and could have already started to fail which means it could be starting to leak which will get worse not better. May not be bad idea to add a few ounces of Sodium Silicate na40 to your coolant with thermostat removed. (buy of net not in stores as those are not the same). Also do a compression test on cylinders and test for exhaust in coolant to diagnose possible head gasket failure.

Thank you very much for your response. I very much appreciate it. It is one year later and after months of no temperature issues(it simply went away) it has started happening again infrequently. I will try your suggestions one by one. I truly hope it is not a head gasket, and hope the infrequent nature may mean it isn't. Thank you again.

Be sure to check your thermostat.

Test for exhaust in cooling system & compressions test all cylinders if head gasket failure is suspected.

Check coolant reservoir for leaks/cracks/pin holes and cap for sealing good.

A block testy if you have access to a air compressor can pin point a leak in the cooling system.

After so many years its best to purchase a new reservoir cap as it looses its ability to seal, since the cooling system is meant to stay closed any small pressure lose will cause issues.

Thank you all for your answers. Thermostat has been replaced already. The mechanic did a pressure test which didn't show anything but had to add a half gallon of fluid prior to the rest. He checked all the fans and hoses but feels confidant it is the radiator. Would a good pressure test normally exclude the radiator?

Different test for finding leaks or issues so mostly include the radiator, some you need to run the engine too.

Referring to block test, I mean checking for compression loss in a cylinder but using a compressor instead of turning the engine over to make compression to test if it has enough. A actual block test I should have stated better is testing for exhaust in coolant at the radiator, so for a block test the radiator has to be in place as the engine will be running during the testing.

The compression test by way of adding your own air, is a way to find leaks in the engine as you can hear the air escaping the engine from testing one cylinder at a time, often this escaping air depending on where the gasket is bad or if the is a crack somewhere can go into the cooling system, which can be done without the radiator as long as you know the rest of the cooling system is up to par. With the cooling system intact you can also find leaks other than gasket leaks, in hoses, radiator or cap ect. if there is a leak from the engine going into the cooling system.

This is the exact intermittent issue I've been having with my 2003 Civic, 148k miles. The temp rose two years ago on a road trip, went back down and had no issues until about a year ago it started randomly spiking and then dropping back down to normal operating temp during a long drive (over 2 hours, I've never had an issue on shorter drives than that). I had my shop put in a thermostat and a new rad cap. They tested for hydrocarbons in the fluid and it was inconclusive (small amount but not over the threshold). It overheated on me recently during a 4-hour drive, took it to a shop, they did a bunch of tests, drives, etc. and couldn't get it do happen again. Put a new rad cap on and drove home, then to have the temp fluctuate randomly again. I purchased a fan sensor/switch in hopes that will fix the issue. I can't imagine how this could be a head gasket... no fluid loss, intermittent, etc. Just purchased a newer Civic as I can't have an unreliable car for longer trips.

So my car does the same thing it’ll rise to 3/4 then bounce between half and full but even when it’s all the way up there’s no significant heat coming from the motor but the fans are still kicking on as if it was and I’ve done the thermostat in the last 6 months topped my coolant off last month and still having the issue