Wedge expansion/header tank.

[wooly350i]

Hi folks, for some time I've been questioning to myself why the stainless top pipe off the thermostat housing which goes to the top of the radiator has always drained once engine has cooled and can this lead to an air lock? If I remove the screwed cap and allow the water to back flow up through the rad the temp gauge in the car behaves but if I let it do its own thing in the hope it will refill when the stat opens, the temp gauge dances up and down quite a bit and looks to get too be over heating. I had my expansion tank made up in stainless some time ago and its occurred to me that the top connection on it may have needed a dip tube dropping to near the base of the tank inside so as the water cooled it would syphon water back up the tube to the pipe i'm questioning and refill it automatically. Can anyone out there tell me if their original tank has a dip pipe on the top connection inside the tank or am I barking up the wrong tree. As they are plastic I know they get clouded with age but any help would be appreciated, cheers for now, John
PS. I hope I've explained that ok.

[Wedg1e]

Mine does that John.

[wooly350i]

Wedg1e said:

Mine does that John.

But is it right Ian ? If the top tube had a dip pipe to at least half way down the tank then as the system cooled I'm sure it would drag water out of the tank and top up the top pipe to the rad as it tried to empty, the big BUT is would the top of the tank have the potential to turn to steam and would the top hose then vent properly although perhaps the pressure would be ample to push any air out the dipped pipe anyway. Suppose I've gotta be the one to try it then!