Coolant temp sensor pegged past H after coolant drain
 

My coolant temperature sensor is pegged past the H and I believe the whole system is bled.
Any suggestions?

Air pocket at the sensor.

Coolant temp sensors cant read temp without being wet, an air pocket will cause them to read max/min



Confirm that it's just the gauge showing wrong, and the head isnt genuinely cooking, but it's probably just a pocket of air.

Bleed again, burp all the hoses, etc.

Burp the hoses by pushing on them right

What? They can absolutely read the temperature of a gas.

Pretty much every coolant temp sensor I've dealt with doesn't read gas temps (well, they show a reading, but it's so far from accurate that it's pointless)

All the ones i've had in cars have always pegged one end of the temp gauge when they're in air.

You made an incorrect assumption based on anecdotal evidence. You've see an unexpected reading when the sensor is uninstalled but still plugged in, and assumed the sensor isn't working.

The fact is, the sensor is just a thermistor. The resistance of that thermistor is going to change based on its temperature. It makes absolutely no difference what the state of the matter is that it's in contact with.

The reason you get an incorrect reading is because a single wire temperature sensor is meant to conduct through the threads it's mounted with to chassis ground. By unmounting the sensor you're disconnecting the circuit that's used to measure the resistance and give you a meaningful reading. Unscrewing it gives exactly the same effect as unplugging the wire.

2 wire sensors are used slightly differently. The sensor used for the gauge in a miata is a single wire.

Since we are talking about burping here, I just wanted to say I got one of these vacuum coolant refill tools and it's much better than using one of those Lisle spill free funnels or other methods. I believe this style of tool is relatively new to the market and I had never heard of it before. The bonus is that it also checks for leaks