So i just did my timing belt and water pump now, when im driving it runs right in the middle but when i am idling or put it in neutral to coast down a hill the temp gauge will drop pegged to cold. what could this be? the heater inside the car stays warm the entire time.

 

Alright update, I was cruising in 5th gear and all the sudden it dropped to cold, down shifted into 4th and it went back up. Im leaning towards coolant temp sensor, is there anyway i can test it?

 

Did you also change your thermostat when you did the timing belt and water pump? It might be unrelated but is worth asking.

 

I didn't change it but, when the tstat is closed the car will overheat correct? and if its stuck open the car wont heat up while driving. The car heats up while driving but then when i stop and idle the temp will drop.

 

It sound's like that gauge is either losing power or losing signal you might want to check if everything down there is connected correctly and no lose wires down there because what ever is causing the problem is only happening when the engine is not working hard enough like when your cruising and idling.

 

yah almost like the sensor is not reading correctly when the coolant isn't flowing as much? idk its quite tricky ill take a look at the wiring connection and make sure everything is solid there.

 

99% sure it's your tstat....that happened to me....how's your heat working? that will really tell you....

 

I dont think its the tstat my heat is perfect, and it never over heats which are the symptoms of a closed tstat but it heats up which is the opposite symptom of a open tstat i think its just the sensor