Ok my cars cooling system is basically all freakin' new. Brand new radiator, upper and lower hoses, water pump and just put in 195 thermostat hoping it would fix my problem. Also the fan is on all the time and I know it's working. When I drive my temperature is all over the place. goes allllll the way up to the red range.....then back down and a lot of times stays there for the entire ride and as long as the car is warm. If the car gets cold though, it does this again. It's starting to get very bad last time I could see the coolant boiling! Also, it's not leaking a drop anywhere!! PLEASE what could this be????? There's nothing left......Thank you!

P.S. my car is a 91' 305 TBI RS

 

Where did you see the coolant boiling, the overflow?

 

Yes in the overflow.

 

My guess is that it is the thermostat. Tha fact that its new doesnt mean its ok. Remove the thermostat and test it in hot water and see if it works ok or not.

 

Are you absolutely sure you got all the air out of the system?

 

Two most obvious are bad thermostat (some are junk right out of the box) and air in system will create problems also. The up and down temps are interesting though as a thought I would just make sure your temperture sending unit is attached completely and working properly.