Expansion tank blown

[andrewnmoore]

Good evening all,

I took my E85 in for an MOT today and the expansion tank blew during the emissions test. The engine needs to run at 3k RPM for 5 minutes during this process according to MOT regulations. The MOT tester, and boss of the garage chain, told me this is not uncommon due to the 'high' RPM it's put under during this test. I'm also told that a faulty radiator/water pump could cause this.

I'm hoping someone can share any information regarding this as personally I think this is bull! Please tell me if my logic is incorrect - I've put 70k miles on my Z4, very often over 3k RPM and that's coming out of 2nd gear. I find it hard to believe today's emmision test put it under so much stress for this to happen! My feeling is the lads who took to my car revved the hell out of it whilst cold and blew it resulting in a cost of £100. Whilst I'm not turning my nose up at £100 I'm more annoyed at the fact there is a possibility someone thought it was OK to sit in my car and give it some beans because it probably sounded nice in the garage!

Please, someone put me out of my misery and tell me that this is something that has a good chance of happening! If so, I'll always remind myself that when taking my car for an MOT is comes with the possibility of destroying my expansion tank!!

Cheers everyone

[GuidoK]

The expansion tank is a known failure point on z4's and bmw e46's.
It cracks over time.
I think 3k rpm revving is not high (in fact, its pretty low), and a cold engine stresses the expansion tank less than a warm engine.

So I think its just bad luck and that the MOT station is not to blame. It was just time up for the expansion tank.
Consider yourself lucky that it happened in a garage. I had it blown in the alps in the middle of nowhere (but if you monitor the level and top it off regularly you could in theory drive it indefinately; the z4 has a level sensor for that tank giving you a warning light).

100quid for replacing the tank is a good deal. Although the tank is not particulary expensive, its a pain to get off as it can be very stuck to the radiator.

[andrewnmoore]

Thank you very much for your response and info. It's appreciated.

This brings me a sense of relief and has lowered my anger level, marginally I guess it really is a case of bad luck that it happened but as you say, fortunately it's in a place where it can be fixed! In all fairness, my E85 has been the most reliable car I've ever owned, even at 10 years old and having had a brand new car in that time too! Perhaps I should let the technician live to fight another day...

[mr nurburgring]

The only reason I can see them needing to rev it up would be to get the cats/engine up to temp to pass emissions.
I wouldn’t worry about it too much they had to rev mine as it took it in and it was cold.

Those tanks are known to crack, I did mine a couple of years ago.

[GuidoK]

^afaik its both temperature and the set rpm to measure emissions.

To get my car pass emissions, they have to make a test drive to get the kats properly up to temperature.

[Mr Tidy]

I think it's coincidental to be fair.

BMWs of this era are known for suffering from split expansion tanks - my E46 got a new one at 80K when the "banana" shaped radiator got replaced.

[andrewnmoore]

Thanks very much guys. I feel reassured

[NickDE]

At the age and mileage it was probably due to split. They really should be treated like consumable items and be put on the list of things to be replaced at regular intervals.