Thread: Thermostat Housing Plug

That is well and truly sad looking.
If you cant get it out then there is a small rubber hose connected to the left of the thermostat housing.
Disconnect the rubber hose and remove the plug from the top of the radiator.
At the other end of the small hose is a 3 way fitting.
Remove the fitting and ensure you can blow air through it in all directions and then refit.
This fitting helps remove air from the system.
Fill the header tank until fluid comes out of the top of the radiator then put the bung in.
Keep filling until fluid comes out where the small hose connects.
Put the small hose back on.


Massaging the bottom radiator hose while filling will help.

I recommend persevering with the top bung. It definitely sounds like an air lock.

It's one of those jobs that is incredibly tedious but really needs doing.

Did what you said jboot51 and the hose that went to the thermostat housing was clogged with some grey stuff



Put in a temp house just to see if I could get it all working



Started it up and same thing temp gauge went all the way to hot even when it was dead cold. The new hose did let the system air out a bit i was pumping the hoses and quite a bit came out. Stopped and started the car a few times till that new hose didnt fill up with air but still no change to the sender, still reading high and the thermostat didnt seem to open cause the top hose wasnt getting hot nor the radiator.

Whats the chance of it being a faulty thermostat ? And also the sender? With the wire that goes to the sender disconnected it reads cold on start. With the wire connected to the old sender but just the sender sitting on the block ( Not bolted in)

If that hose is moving the coolant fine then is there any point trying to get that brass plug out?

How long did it take for the temp gauge to go up to hot? If coolant is flowing correctly but the thermostat is closed you could idle it forever and not get the gauge to read hot. You need load on the engine to warm it up.

If it is shooting up to hot very quickly, then you either have a faulty sender/gauge or the coolant is not flowing correctly. Eg. Air lock still.

Could the heater core / pipework be blocked restricting flow? Eg. more grey stuff?

with the engine running, even from dead cold, there should be coolant flowing through that new hose you put on, maybe pick the revs up a little and any air will make its way to the header tank.


The 3 way connector and the other 2 hoses need to be clean aswell


It takes a good 10 minutes at say 2000 rpm to open the thermostat on a 300 tdi.


Faulty sender would be my guess


You can take the thermostat out and have a look, no gaskets required as it has a rubber sealing ring.
Test it by putting it in boiling water.


You've got the clear expansion tank, on mine from cold, with a few revs, the coolant flows up from the little hose and hits the roof of the header tank.

That grey stuff looks ugly-some sort of stop leak maybe???

Originally Posted by Judo

How long did it take for the temp gauge to go up to hot?

The gauge hits its hottest point (Off the charts) the second you turn the key. Might put the old sender back in tomorrow run it for 10 minutes and see how we go.