Odd coolant leak - advice needed

ryushe

Member
 Rotterdam, NL
Evening!

Yesterday I noticed a puddle of coolant fluid underneath the car. Checked the fluid level and it was far below where it should be, so topped it up this morning (I had some in storage) as I had to use the car and no real time to check for where the leak was coming from. Took the coolant I had spare with me, drove to the office without any issues (no lights, dead perfect on temp, no odd fan behaviour) , and when I got back to the car this afternoon there was a puddle again!
Office garage has pretty good lighting, so had a good look at the hoses and such, but don't see anything obviously leaking. Water pump seems fine, didn't feel anything leaking at the bottom of the pump. Expansion tank *seems* fine.
That said, there clearly was a puddle of coolant again on the floor, and I also noticed a small puddle in the undertray. So topped it off again and drove home. Opened the cap and fluid level seemed fine. Watched for a few good minutes to see if I saw anything dripping, didn't see anything so left it. I've had it sit for the last couple of hours to cool off and had a look just now again, and the puddle has returned. Still don't see it actually dripping fluid.

Anyways, clearly I have a leak. So trying to work out my best bet to further investigate tomorrow when I have some time.
I'm thinking it can't be the pump as the car comes up to perfect temp and stays there. Hoses where dry, so I think I can exclude them too.
Last thing I can think of is the expansion tank.
Does this seem logical? Is there anything else I can look at before just ordering parts and spending a nice Saturday replacing things?

Car has done close to 200k km (125k miles). As far as I know, the pump and thermostat were replaced around 120k km.

If I do replace any parts, anything I need to look out for in terms of special procedures? I assume I'll need to drain the system.
Appreciate any insights!
 
The expansion tanks are devious little sods

They crack and either let steam out or I’ve seen them leak when cold and not hot

For the sake of about 50 quid I’d say get a new tank on it and inspect the hoses around it
 
Also, while the system still has pressure in it, try wiggling the top hose. I had one where the seal on the engine end was seeping. wiggling it let more water out. Of course, the water then ran down the pipe, dripped onto the auxiliary belt and was flung all around the engine bay, mostly disappearing without any idea where it had come from.
 
With the engine warm, shine a torch vertically downwards into the cavity around the expansion tank. You might see a micro-jet of coolant that looks just like a spider thread, barely visible. That’s what happened to me. When I removed the expansion tank, there was no crack or hole that I could see, but it was definitely there.
 
Thanks everyone for your input!
@enuff_zed, good idea. I checked what you mentioned, seems completely dry.

In the hope that it's the offending item, I ordered an expansion tank and associated hardware, hopefully it'll be here soon so I can fit it this weekend. Hopefully this fixes it. Will report back.
 
If that didn't fix it, coolent die could help trace where it is coming from.

Coolant System Leak Detection Dye - [RA-RLD2]

You will also need a UV torch, can get online for a fiver :)
 
Jack the car up, place axle stands and remove the front undertray panel. From underneath you get a good view of all the pipework to check for leaks
 
Mine was expansion take, had a pin hole to start, would come and go, noticed a bit more coolant as it went on, had it checked about 3 times, pressure was always good. In the end it obviously got worse and worse.
 
Update, this was fixed. Turned out the top nozzle of the expansion tank (circled below) was quite literally crumbling inside the hose, and thereby leaking all over the place. New tank fixed everything.

bmw-tank.png
 
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