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Discussion Starter · #1 ·
I recently have come across a coolant leak on my 2003 e46 325i. It only leaks once it’s built pressure. After taking a look from underneath the coolant is quite literally everywhere. And the highest point of the leak is on the upper radiator hose. I replaced the hose, still the same issue, I replacing the housing and thermostat that it connects to, still nothing but the only obvious area it is leaking from would be the seal of those two. When the engine builds pressure, as I’ve taken it for the odd drive after replacing parts to see, it leaks coolant very very quickly, coolant light comes on within 10 minutes of it running. Has anyone had this problem or has any ideas for where the issue might be?
 

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And the highest point of the leak is on the upper radiator hose. I replaced the hose, still the same issue, I replacing the housing and thermostat that it connects to, still nothing but the only obvious area it is leaking from would be the seal of those two.
I don't think the new hose is leaking at the Tstat housing connection. The hose O-ring seal cannot leak this way. Did you hear the click when the locking tabs of the hose snapped in the groove? If hose was not inserted deep enough then it can leak.
The Tstat has rubber gasket to the head. did you clean up the flat surface on the head where the gasket sits?
 

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Discussion Starter · #3 ·
I don't think the new hose is leaking at the Tstat housing connection. The hose O-ring seal cannot leak this way. Did you hear the click when the locking tabs of the hose snapped in the groove? If hose was not inserted deep enough then it can leak.
The Tstat has rubber gasket to the head. did you clean up the flat surface on the head where the gasket sits?
Yes it clicked and locked into place. I did not clean the surface but it looked very reasonably clean. On another hand would it be possible that there is a leak coming from the expansion tank and the belts are driving it throughout the car?
 

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Yes it clicked and locked into place. I did not clean the surface but it looked very reasonably clean. On another hand would it be possible that there is a leak coming from the expansion tank and the belts are driving it throughout the car?
Oh, so you didn't really see it was leaking where the hose connected to the Tstat, but you just speculated. Open the hood, remove the fan, have the engine running until coolant leaking then look to see where is the leak.
 

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Rent a cooling system pressure tester.
 

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I recently have come across a coolant leak on my 2003 e46 325i. It only leaks once it’s built pressure. After taking a look from underneath the coolant is quite literally everywhere. And the highest point of the leak is on the upper radiator hose. I replaced the hose, still the same issue, I replacing the housing and thermostat that it connects to, still nothing but the only obvious area it is leaking from would be the seal of those two. When the engine builds pressure, as I’ve taken it for the odd drive after replacing parts to see, it leaks coolant very very quickly, coolant light comes on within 10 minutes of it running. Has anyone had this problem or has any ideas for where the issue might be?
Sounds like it’s coming from the o-ring on the upper radiator hose.

Could be at the 6 o’clock position which makes it difficult to see. That assumes the leak isn’t coming from the radiator cap itself.

Photos of the leaky parts might help.
 

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There is no radiator cap here. Where did you see a radiator cap on e46 325i engine?
I think we know what he means.

I'd be having a close look at the tank, given their habit of cracking. That would be consistent with the leaking more under pressure. Any idea how old it is?
 
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