Hyper_545i

I Could be wrong, but I am pretty sure the opposite is true. If you open the cap on the resevoir and let the pressure loose, more coolant will spill out.

Hyper_545i

Here you go.

turboawd

obviously you'd want to change the sensor when the engine is cool. but i'd definetly open the cap to let any pressure that may still be present out. then i'd put the cap back on, then change the sensor.

BimmerFan52

There is no advantage to first removing the cap and then replacing it, but there is an advantage to leaving the cap in place when removing the coolant sensor as less coolant will be lost.

If the system is sealed, the cycle of heating and cooling the coolant will not produce more air (or a higher pressure) than was originally in the system when the cap was last off, once the system returns back to a cold state. If all of the air was driven from the system properly when it was last filled with coolant, all of the air in the system will be at the top of the coolant overflow tank.

The air at the top of the coolant tank starts at atmospheric pressure and is then compressed once the engine reaches operating temperature and the coolant expands, and is then compressed even more after the hot engine is shutdown and coolant is pushed into the overflow tank (from the radiator through the vent pipe) because of the heat soak from the engine block that super heats coolant in the block passages and creates steam and increased system pressure. If the coolant overflow tank has become weakened or brittle with age this is when it will burst, usually at a seam.

Once the coolant in the block and radiator cools and shrinks, the higher pressure air at the top of the overflow tank pushes the extra coolant it received back through the vent pipe to the radiator, returning the air in the coolant overflow tank to atmospheric pressure.

Leaving the coolant overflow tank cap in place while removing the temperature sensor in the lower radiator hose will have an effect similar to pushing a straw into a glass of water, then capping the end of the straw with your finger and then withdrawing the straw from the glass. Given the sealed system, the surface tension of the liquid at the small opening left in the hose once the sensor is removed will slow entry of air into the system and will cause the coolant to drip relatively slowly.

insyder

You can buy just an o-ring.

I followed this video's example and went with a home depot o-ring.


ECS Tuning also sells the "proper" o-ring

pcy

I went thru this song and dance before...
I replaced the o-ring on that temp sensor. Leak continued.
Then, I replaced the temp sensor (new o-ring was on the new temp temp sensor). Leak continued, albeit slower leak.
Then, I replaced the lower radiator hose and installed the temp sensor on the hose. That fixed the leak.