Coolant keeps overflowing out of expansion tank

[MrGCC]

Hi everyone,

Needed a little bit of help and hope you all can help. Car is a 1992 318I auto with the M40 engine.

So recently changed my thermostat, water pump and everything went fine. Since the change I have bled the system and have had the hot air coming out the interior. Works fine and this has been the case over a few day that went the car gets all the way up to temp it starts spurting out coolant from the expansion tank cap.

The water was filled to the cold mark before hand so I have again bled the system and tried again. Same thing happened.

I opened the expansion cap while running fairly cold but warming up to see if any air bubbles were coming through (checking for head gasket) but nothing, so I brought a new expansion cap from BMW and installed that. Thought everything was fine until I took it for a little drive today and same issue again!

I just cant figure out what the issue is, no oil mix in the coolant either. I understand that the coolant also flows to the transmission being an auto so could their be an airlock all the way to the gearbox?

I'm just a little stuck trying to figure out what is causing the coolant to flow out the expansion cap constantly when the car is up to temp!

Thanks in advance everyone.

[mark_i]

2 things to keep in mind with an M40 cooling system

1/ the level of coolant you observe in the expansion tank can vary. i have had systems where the level is well below the cold mark and all is fine.
2/ the M40 has two bleed screws. one on the expansion tank, the other on the 'stat housing. double check both when the engine is up to temp.

[MrGCC]

2 things to keep in mind with an M40 cooling system

1/ the level of coolant you observe in the expansion tank can vary. i have had systems where the level is well below the cold mark and all is fine.
2/ the M40 has two bleed screws. one on the expansion tank, the other on the 'stat housing. double check both when the engine is up to temp.

Thanks, I really think that the coolant must have an airlock in it still, and once I get the airlock out i'll see what the coolant level settles at. Just very weird as I do think the cars system was bleed fully when I got it all finished

[martauto]

The coolant does not flow to the gearbox.
The gearbox fluid flows to the radiator to be cooled.
So you may just have an air lock mate!

Mart.

[BHadley]

Park car or raise up so front end is higher than rear
Run car
Locate hoses going to the heater matrix
Remove bottom hose until coolant spits out
Reconnect
No more air lock

That's from memory anyway

[MrGCC]

Thanks for all the advise everyone, I haven't had enough time lately to get to the bottom of this but hopefully will know the answer within the next week of what the issue is.

Still got a little digging around to do!

[arrisbmw]

is the rad new ? why did you change things ?. you must be getting a lot of pressure build up in the rad to cause the cap to blow fluid out. if you got a small crack in head gasket that could pressure up the cooling sys. hopefully it not that.