Too much coolant pressure?

[ZsZ]

Today my rear heater matrix gave up. (Soaked all the back and middle, big mess, lot of carnage.) No visible hole, but blew at least 10 litres of coolant under the bench.
Last year I had to change two coolant hoses as the originals split.
I had no overheating problems since I put in the Tdi.
2 hours highway, ~4000rpm, needle just over the LED and bang. The hoses gone on similar highway runs. 
My suspect is the relatively new Blau coolant cap. (3-4 yrs old). Is there a way to test it?
Before that I had the crappy two valve cap which I never had problems with. I just thought the original style is better.

[R0B]

[ZsZ]

THX! Will check it tomorrow.

[ZsZ]

OK, tested it.
It trumpets when I blow, but I can suck through it.
Disassembled the cap, it seems like the fingers that shpuld keep the springs together are just not catching enough




and there is anice crack in the heater matrix

[ZsZ]

I was wondering how could the pressure build up when the cap was not holding pressure.
Or I miss something?

[Robsey]

Further to that - how can it build pressure with a split heater matrix?

Any other vehicle I would query blow-by gasses or a paggered head gasket.

I am not experienced enough with wasserboxers and diesels to know if there is any difference in "the usual suspects".

For other vehicles, the usual first line of diagnosis is to do a coolant bottle sniff test for ambient exhaust gasses.

[ZsZ]

Yes sniff-test is on my list.
Will do after my hols. (We broke down on the highway headig to a 2 week croatia trip fortunately relatively near to my moms house)
Highway with high rpm is what it does not like.
Will buy a new cap too - probably some crappy two valve will be in stock at the local FAPS.

[ZsZ]

After the SOS repair (VanWesel branded chinese radiator, Japanparts branded EU+ coolant cap with the crappy two valve design) we made ~700kms to croatia (no highways)
The bottle behind the licence plate still has the clear distilled water I put in after the bleed.
I am not sure if this is good or not.