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Alf McQueef

Tyre pressure weirdness

I was helping Lu learn to roller-skate on Saturday and noticed my NSF tyre looked a little soft - it was on about 24psi (as opposed to 36 normally). I had run over a plank on the M25 the day before and assumed I had picked up a puncture, on a 2-week old winter tyre.

This is annoying as I would never know if I had a slow puncture except the old fashioned way. The TPMS in the Jaguar is erratic at the best of times, since switching to the winter wheels (despite them being the same wheel/tyre sizes and with the right TPMS valves) there has been a massive TPMS system error, which seems par for the course from forums - I think it needs resetting by a dealer.

However I put it back to 36psi, and its still showing 34+ now, the difference is probably just measurement/temp related. I checked them all when I got back from the dealer with them fitted 2 weeks ago, what the hell is going on? Can tyres - or these stupid big TPMS valves - take a while to seat? All the slow punctures I have had in the past were determinedly steady and usually terminal for the tyre, with a nail or thorn well embedded in it. It has not been driven since I put more in, but I can't see that making a difference.

Weird - and, given the TMPS not being fit for purpose, annoying...

Have you checked the tyre as well as you can for a nail or screw?

On my last car I had a cracked rim which hardly let any air out until I had a bit of a spirited drive at which point it would lose 3 or 4 psi. I could go a week and only be 2 or 3 psi down but if I had a bit of a blast it would lose nearly double that overnight.

I thought it was a problem with the tyre sealing properly until it failed the MOT!

It's possible that the tyre wasn't seated properly.  Assumihg the valve is OK, might the wheel itself be becoming slightly porous?  

I had a slowly-deflating rear left tyre earlier this year but after having all four wheels refurbed on the Cayman (original finish was flaking off, they remain completely unkerbed) and the tyres refitted, it's been fine since.

The left rear on the Boxster is going down slightly, but only 1-2 PSI every couple of weeks, so not really worth doing anything about as they'll be swapped over in about a week.

If you have the Jag TPMS reset (well the units paired with the car) by a dealer, then in future the car will recognise those TPMS units. So with all "paired" units, swapping between summers and winters or swapping tyres across axles will not give an error.

I have been told that slow punctures / sidewall / rim damage sometimes only show when you park the car with the damaged area at the bottom, so all the weight is on it.  

So a tyre that goes down slowly and is then pumped up, may stay inflated until it is put under full parking weight pressure again - when it will suddenly go down.

How old are the wheels, corrosion on the rim can lead to slow air leaks. A refurbished on my ZT260 wheels cured a similar problem. It was irregular, too. Some weeks it was fine other weeks it would go flat over 3 or 4 days.
Alf McQueef

The wheels are immaculate, two were refurbished from quite poor condition, two (including this one) are original with barely a mark on them. They have been sitting in a pile in my garage for months so given the time I'm in there training and fiddling with RC cars, I have had a good look at them! I polished them too.

I've had a look and feel around all of the tyre I can see, I'll do it again after moving it. If nothing more happens then fine, it just seems very suspicious... I'm assuming a cracked rim would be fairly obvious, but maybe it is not unless you have it refurbished...

I had similar on the A2 which kept periodically reoccurring, turned out to be a poor seal between tyre and rim. 15 fix at the local decent independent tyre place and all was well.

I would think having the tyre re seated might be worth a try.
Alf McQueef

Cool thanks - it is still on the same psi after 3 days (thought has still not moved). I'm going back in to the place that fitted the brakes/tyres at some stage to have the brake wear sensors fitted - it its still playing silly buggers I'll get them to refit that one.

Thanks for the TPMS info PG - there are far fewer XF's in the world than things like 3 series, and they seem less internet savvy - opinion varies on a lot of technical topics! I can say that on the summer tyres, about a quarter of the time it shows each tyre in turn being flat, its not like the BMW system where as soon as it showed a problem I stopped and checked (once it was a puncture, once it was not).

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