Permanent oil cooler vs thermostat

[Lt86]

(03-24-2015 07:35 PM)Bean Bandit Wrote:  have you made sure the system is free of any air? sounds to me like you got some air in there... only reason I could think of for the gauge to raise that quickly (except of being faulty itself) - although you normally would see inconsistencies as well as the air is pushed around.

I bought a brand new sensor and it had this issue from the first minute. You start the car, let it idle and it almost directly rises to the normal 'hot' temperature. It doesn't go over the middle point but the temperature does seem to drop below sometimes when the engine gets cooled more.

I'm pretty sure the system is free of air. How do you get all the air out if air would get trapped behind the thermostat? I can't open the system unless it is cold.