Coolant leak - filler cap seal?

A

adam04

Member
 Manchester
Opening up the bonnet a few weeks ago, I noticed much of the plastic covered in white spots, almost similar to salt drying but its over the top of the engine cover so unlikely to be that. On closer inspection i noticed it was concentrated around the coolant top. I put it down to the cap not being correctly fitted, topped it up and cleaned away the evidence.

I've not been out in the car much lately but noticed today that again, there seems to be spots of white around the filler cap, pics below.

I had the expansion tank replaced about 6 months ago so this seems an unlikely cause. My next thought is the seal around the cap and to replace the cap, any other thoughts what this could be?

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Hi Adam. Order a new cap and try that. It certainly looks like it is the cap venting for some reason. My water pump bearing seals went recently and that made a mess all around the engine bay as it was leaking onto the belts and then being sprayed everywhere :x If it still does it with a new cap then you have a pressure/temperature problem of some kind or it is leaking from somewhere else. What does your temp gauge show when your driving ?
 
Go out for a spin and then lift the bonnet and have a look, hopefully you should see what is happening. But DON'T remove the cap when hot :wink:
 
Tried that and nothing obvious spitting out. Hence it seems it must only be when its at high revs/temp/pressure.

Ordered a new cap so will see if that fixes it
 
New filler cap, no white spots anymore but coolant is still going down. Only evidence i can see is a pool of dampness on the tray below the radiator/engine. Ive checked for leaks around the various bits and cant see/feel anything so must be further down.

Off to the garage next week to hopefully find and fix.
 

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Hi I had the same issue I was told it could be 2 things water pump or expansion tank in the end it was expansion tank it was split.
Cheap to buy hard to do not much space to work with also you have to bleed the system. Local garage done the job £50.
Hope you get sorted.
 
[ref]adam04[/ref] as [ref]Mr roadster[/ref] says, the expansion tank is a common failure in BMWs with these expansion tanks. Hairline cracks (that are hard to see) weep coolant under pressure. Some say the job is fairly straightforward (under an hour) but the ease/time depend on the individual, of course :)
 
Op says he fitted a new tank 6 months ago. Still, could be the new one split again I suppose...or maybe connection to it. Water pump is other possibility as mentioned above. Also these cars are getting to the age now where radiator leaks are possible due to corrosion as is temperature sensor o-ring on bottom hose.
 
Just to update this if anyone comes across it at a later date, well it took a fair amount of investigating. Initially it was a very minor leak requiring a top up now and then, had system looked over and pressure tested with nothing obvious so initially put it down to the colder weather at the start of the year and possibly some bad seals that were expanding when hot. This continued for a while with a top up every few weeks.

Roll on a few months and a few weeks ago i began topping up much more often, it was needing a top up every couple of days and seemingly always the a similar amount (about 750ml) pointing to something towards the top of the system. I also stuck a UV dye in to see if I could spot anything.

Up on the ramp at the garage (nothing to really spot from up top), the thermostat housing was leaking. The seal was virtually flat to the point you wouldn't have known it was actual rubber and not part of the plastic casing. So this was replaced, alongside the water pump for some preventative maintenance.

All now seems to be happy again.
 
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