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Over heating water temps and an inoperative air conditioner/heater

So here are the symptoms: I was driving home from work (night shift) and it was the first morning that a heater was necessary (maybe 50 degrees?). I turned on my heater when I thought the water temperature was high enough to blow warm air. I only got cold. Then I looked down and noticed that the water temperature was almost to the red line. So I turned the heater off, it immediately dropped. I turned the heater on, and hot air came out. Temperatures rose and on a 90 degree day, after blowing cold air for about 15 minutes, suddenly the car was blowing hot air and the water temp (if I recall correctly) was rising. I am now seeing this on a daily basis. I am trying to keep my windows rolled down, but I am noticing my water temp is running high-ish (a dash or two below the red line). I don’t know if this is related, but last winter when the car was cold the car idled inconsistently. Unfortunately the problem couldn’t be duplicated to a mechanic because by the time I got there, the car was warmed up and idling fine. By inconsistent, the engine would rev up and stop, as though someone were gently pressing the accelerator then releasing while the gear was in neutral. The car is a manual transmission. It has been suggested the problem is the thermometer. Is this correct? Can I fix this myself or what does this run to have a mechanic do it?

Sounds like you have air in the cooling system. You should have somebody replace the thermostat, bleed air out of the system, and check for signs of a head gasket problem.

What year and how many miles on the Corolla?

2004 and about 120k miles on the car. This would be the first non-routine maintenance on the car.

You could have either a bad thermostat or a water pump failing. At your cars mileage it would be a crapshoot to guess. However a GOOD mechanic could help you decipher it.

Thermostat replacement is about 150 a waterpump is about 400

Thanks, the timing belt apparently is a chain and was checked last winter, maybe 15k miles ago. In fact, that was checked in regards to the funny idling (though, it was suggested just a fuel injection cleaning - if I am quoting that correctly - would take care of that). I will look into, I know a pretty good place to take the car

This will not fix your problem but I believe you might be able to add coolant to your vehicle to get you by for awhile. If you don’t know how to do it properly you can get air bubbles in your coolant. Then I would have it checked for a leak or a stuck thermostat.

It is very serious not to run your engine when it gets overheated! It can and will cause severe engine damage!

Thanks all! As it turned out, I was extremely low on the coolant with some leaky hoses. It was interesting getting to the mechanic, pulling over and shutting down about every 1/2 mile!