For about 1k-2k miles I've been having an issue where my temperature gauge will randomly drop to the bottom of cold, it never overheats.. only goes up to normal operating temperature and sometimes drops and goes back to operating temperature again, sometimes in park it'll idle up to 2k rpm for 8-10 seconds before settling down again. Gas mileage seems worse, haven't noticed black smoke out of the exhaust and only other issue was the other night cat almost didn't start... Gave it 3 cranks for 3-5 seconds each before finally the 3rd time it started.

Am I correct that it's probably the coolant temperature sensor located under the intake? TIA

 

Nobody knows if that's your issue, but it's a good place to start. Remove the sensor and measure if the resistance in different water temperatures is in spec, also check for wiring issues between the sensor and the ECU.

 

Did you replace the battery recently? If not still check your terminals for a tight fit and clean from corrosion. A loss of battery power will make the computer lose learned memory and cause idling issues etc. If you did replace the battery and or find a bad connection, the throttle will need cleaned out to bring the idle back into control for the computer. A loss of power or ground can cause any number of issues including a gauge falling out and in. So check your grounds on the engine and chassis as well.

 

I had a similar issue a couple years ago - operating temps low to normal, back to low, up and down but never overheating. Based on the code I pulled, it was either going to be the sensor or the thermostat. I replaced the sensor since it was easy to get to and relatively straightforward to do. Unfortunately problem still remained.

I then checked the thermostat. It was messed up, rubber components torn and bent around it, etc... I replaced it and problem solved.