Random Specific Deflation

[spinaltap]

Last August, I had my local Mercedes dealer replace all the tyres on my Wife’s W169 (A-Class). She typically drives less than 5K miles per year.

All was fine until last December, when the front near side front tyre deflated by exactly 7 lbs, causing the car’s deflation alarm to trigger. I had Mercedes check the tyre. No obvious problem was evident. Fast forward to Easter 2018. The same issue arises on the same tyre: deflation by exactly 7lbs. Again, no problem reported by Mercedes on inspection. I had them replace the tyre valve, just to be sure.

Yesterday, the same issue manifests itself again. The tyre deflation alarm triggers while driving. The same problematic tyre shows deflation by exactly 7 lbs. Weird, right? Why just by 7 lbs in each and every occurance?

In talking to Mercedes, my options are - replace what looks to be a ‘problem free’ tyre (since none of their usual tests reveals any problems). Then, if the same issue subsequently arises, I will need to replace the alloy wheel.

Given the car’s low mileage, and the relative newness of the tyre, I’ve also asked Mercedes to submit the present offending tyre to Continental for testing. IF they discover a problem, they are likely to proportionately refund the cost of the tyre.

 

[renault12ts]

Could be a leaky rim.

Put a can of Holts tyre weld in,,,that'll stop it.

 

[MancMike]

The reason tyres deflate, then stop deflating is because with reduced pressure, the already tiny hole size closes up and doesn't let any more air escape, or the rate at which it's escaping drops right down to imperceptible thereon.

You'd likely find if you loaded the car up with sand bags, that it'd lose 8 or 9psi before stopping.

To find where it's coming from, pump the tyre up to high pressure, whatever it says on it is its maximum, probably 50/60 psi, then spray soapy water all over, it should foam at the leak.

As above, tyre weld should plug the hole.

 

[spinaltap]

Update...

Back to Mercedes today. I was prepared to have the tyre changed (£112), and, potentially, in the future, a replacement alloy wheel (£186).

The same technician checked the existing tyre/wheel again. Pumped up to 60psi (twice the normal psi) revealed two sets of air bubbles emanating from between the tyre and the rim. On removing the tyre, rust was discovered on the beading on the whole of the inside rim. This is considered to be rare on Mercedes’ alloy wheels.

They reasoned that the amount of rust was sufficient to cause a gap between wheel and tyre. Given the number of repeat visits with this issue, Mercedes refurbished the alloy wheel to eliminate the rust, added a new valve, and applied sealant between wheel and tyre at no cost to me.

FWIW, Mercedes don’t recommend the application of tyre weld in this situation as, once applied, is difficult to remove.

 

[GLK]

Good result!

I had a slight deflation on couple of wheels on my A200, before I had the alloys refurbished in November 2016. And yes - they were slightly buckled.
Since then - not a thing. Still monitor the pressure (now via TPMS), but all four now hold it perfectly, hardly changing in three to four weeks.

 

[MancMike]

Good outcome.

FWIW, you won't find a tyre fitter who'll recommend tyre weld. It doesn't mean it doesn't do its job though.

 

[Dryce]

Why just by 7 lbs in each and every occurance?

Is that not just the threshold point at whch it is detected and the alarm triggers?