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.

[Jeffnhiscars]

A blown head gasket can cause exhaust gas to escape to the coolant. There's a test strip you can buy to test the coolant for signs of exhaust

[MrGcc]

A blown head gasket can cause exhaust gas to escape to the coolant. There's a test strip you can buy to test the coolant for signs of exhaust

thanks i'll look into that, I thought by looking at the coolant with the cap open it will show if the gas is leaking through with air bubbles but none of that is happening.

[locknload]

Was there an overheat event that prompted you to change the pump/thermostat or was this just routine maintenance? May also be a faulty or improperly installed thermostat (even if it is new). I think I'd also try running it without the thermostat (not permanently, just a test run) to see if there is a change.

[1843lamp]

I noted that you checked the coolant for oil but did you check the oil for water and a higher level?My fathers car overheated yesterday for no apparent reason and there was water in the oil.

[MrGcc]

Was there an overheat event that prompted you to change the pump/thermostat or was this just routine maintenance? May also be a faulty or improperly installed thermostat (even if it is new). I think I'd also try running it without the thermostat (not permanently, just a test run) to see if there is a change.

The car was having an issue where if I was parked up for too long the temp would start going up, so first I changed the fan clutch which didnt fix it, then I moved to the thermostat and water pump. What difference would it make to run without a thermostat?

- - - Updated - - -

I noted that you checked the coolant for oil but did you check the oil for water and a higher level?My fathers car overheated yesterday for no apparent reason and there was water in the oil.

I actually changed the oil today as just maintainance work, wanted to clean the oil feeder above the cam. That was all fine the oil looked normal and old. No water signs in it at all.

[richardodn]

BTW, coolant doesn't run to the automatic transmission. Transmission oil runs to the radiator.

[aaverhulst]

I had a similar problem but the car had a tendency to run hotter at slower speeds. The radiator was plugged. Another radiator took care of the problem.

[locknload]

If the problem goes away after removing the thermostat, you probably have a faulty thermostat or it was installed improperly. (I don't think it's possible to install the thermostat backwards, but stranger things have happened).

[thejlevie]

It sounds to me like you have a head gasket leak that is pumping air into the cooling system.

[ELVA164]

A blown head gasket can cause exhaust gas to escape to the coolant. There's a test strip you can buy to test the coolant for signs of exhaust

It sounds to me like you have a head gasket leak that is pumping air into the cooling system.

Yep.

[90bmw325i]

An inexpensive place to start is the radiator / coolant reservoir cap. If the cap is bad and will not hold the specified pressure, coolant will exit the system under the pressure that the water pump makes while running. I actually ran into this on an Audi A6 once. I thought it was headgaskets at first, but saw no other signs to prove it out. Tested the cap and it held 0 PSI


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[mclaaron]

I bought a generic expansion tank a while back which didn't seal right. Just a thought

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[actletpone]

A compression test may provide some answers. Or test the coolant as mentioned above, think it is called a hydrocarbon test.

[ELVA164]

If we still don't have an answer, has anyone mentioned pressure testing the coolant system?

[E30Garage]

Warning: I just discovered that photobucket is now charging users $399 per year for pic hosting which allows you to embed images, if you do not pay them the fee this is what you see.

[MrGcc]

Hi everyone,

Just thought I would update the thread with my finding, so I eventually found out it was the radiator that was causing the issue..

Did a little YouTube vlog on it all to show the problems etc, so hopefully that will be helpful to someone else at some point. Who knows!

Thanks again to everyone who did help though!

https://youtu.be/d3zFrA5HT9w