Runflats and Tyre Pressures

UK forum for general and technical discussion about the Z4 roadster and Z4 Coupe
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Dur747
Joined: Sat 05 Feb, 2005 19:39
Posts: 38

  Z4 roadster 2.5i

Runflats and Tyre Pressures

Post by Dur747 »

Hi All - hope someone can help with my tyre issues - I have a 2.5i Z4, 17in runflats.....I had a whole set of new tyres replaced at the last servicing in November 2010 and last week I got the dreaded 'run-flat indicator' warning light. This time I measured the pressures - 130kPA in the passenger rear tyre (normal 230kPA), 220kPA in driver rear, 205kPA in the passenger front (normal 210kPA), and 210kPA in the driver front.

I inflated all the tyres to the recommended pressures - front 210kPA and rear 230kPA, reset the tyre pressure monitor and just drove.

It then sat on my drive for 4 or 5 days, I checked my tyre pressures again, and all were normal apart from the passenger rear again which had gone down to 190kPA (the runflat tyre monitor warning had not come on). I re-inflated this tyre to 230kPA and drove yesterday. Today, I checked the air pressure and it was still 225kPA so it seems to have held its air??

My questions are:
1. If there is definitely a puncture, why is that tyre still holding air pressure or is it just a problem with the runflat pressure monitor?
2. Is it safe to just inflate up to normal pressures and continue driving?
3. Is the runflat tyre monitor actually accurate in detecting a puncture, or does it sometimes just detect changes in air pressures - i.e. can it give false alarms, or is it accepted that if it does come on, you definitely have a puncture

Any advice would be most welcome....
Zed Carer
Joined: Thu 13 Dec, 2007 20:27
Posts: 1642

  Z3 roadster 2.8
Location: Wakefield

Re: Runflats and Tyre Pressures

Post by Zed Carer »

Dur747 wrote: My questions are:
1. If there is definitely a puncture, why is that tyre still holding air pressure or is it just a problem with the runflat pressure monitor?
2. Is it safe to just inflate up to normal pressures and continue driving?
3. Is the runflat tyre monitor actually accurate in detecting a puncture, or does it sometimes just detect changes in air pressures - i.e. can it give false alarms, or is it accepted that if it does come on, you definitely have a puncture

Any advice would be most welcome....
Hi - not sure about 1. and 2. but if the tyre is only losing a small amount of pressure then inflating back to normal pressure and regular checking should be OK.

As for 3. IIRC the system on the Z4 does not measure the actual pressure in the tyres. When a tyre loses enough air the diameter effectievly reduces and the rotational speed will be different to the fully inflated tyres. The ABS sensors detect the difference in the rotational speed and trigger the puncture warning. That's why its a press and drive reset process to allow the ABS sensors to reset themselves.
Hers: Z4 2.0i Sp --------------------- His: Z3 2.8
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Dur747
Joined: Sat 05 Feb, 2005 19:39
Posts: 38

  Z4 roadster 2.5i

Post by Dur747 »

Thanks for that, helpful...will have it checked out!
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