cooling system header tank

[ChrisG]

This is for my 1985 80 Sport. OK other than a good cleanout and flush I have done all the bits on this, BUT the low water light now flashes continuously.
It should flash when the ignition comes on, then go out unless the coolant level is low when it flashes ( I understand it also flashes if the temperature is very high but as I have a working guage I can see that it is not that.)
Fan cuts in and out as it should.

I suspect the sender in the bottle may have broken ( but was OK before I started draing down , rinsing, new stat etc)

The plug underneath the bottle appears to be designed to unscrew but doesn't.
Any ideas how it comes out?
Is the sender replaceable even?
Otherwise anybody have a working spare
857 121 407
857 121 403
443 121 403
are the part numbers on the bottle.
I suspect this may be used on a range of Audis ( and what's with the 443 number ? )
Thanks !

I've also asked on Audifans.

Guys get out your power steering details as I'm changing the fluid next !!

[mikes2]

got a spare tank with no sender but there's no threads on it.

I had something similar on my quattro after refilling the system - the low water warning would flash instantly.

I topped up with more G12+ and left it 24 hrs and it recovered and it's been fine since. Double check the plug isn't loose

[ChrisG]

OK, I'll recheck the plug later
It's sat overnight, currently SORN so I cannot take it out for a run to check it out, just sitting outside the garage idling away is not quite the same !

Thanks Mikes2

[Martc]

Did you do this to ensure you don't have an airlock?

Slide the heater colntrol to hot.

Remove the bleed screw from the top of the radiator.

Remove the 3 screws holding the expansion tank and lift it up as far as the hoses will allow whilst topping it up until coolant flows out through the bleed hole in the rad.

Replace bleed screw

Lower and replace expansion tank.

[ChrisG]

done the heater thing, it's a DZ engine and no rad bleed but loosened the block temp sender, no air.
Not done the header bottle thing but shouldn't need to with no air in system??
Can but try !
Thanks.

[Martc]

The reason you lift up the header tank is because its at the same height as the bleed screw, so you need the added height to push the air out of the system.The purpose of the bleed screw at the top is to ensure there is no air trapped at the top of the engine block. Not sure if the DZ had a bleed screw on its original radiator (again I don't know if you have an original rad either which may not have one)

How far down the block is the temp sensor, if its way down the block then you could have air in there?