I've got a weird problem and am wondering if anyone can make a suggestion as to what might be causing it. For the last couple of months, all my tyres are losing pressure down to the same value and then no further.

I fill them at the garage to 35 psi. The computer in the dashboard confirms this pressure for all 4 tyres and shows 35 as the recommended pressure.
Over the course of about a day(maybe 2) all four tyres drop to 33 psi which is enough to trigger a warning on the dash to check the pressures.
The tyres will then sit at 33 psi and not lose any further pressure. The longest I've left it so far is a fortnight but I'm not seeing anything to indicate that they would lose any more pressure if left longer.

I spoke to a tyre centre who initially suggested the pressure sensers but feel this is unlikely as it's affecting all four equally and started on all four at the same time. Most likely is that could be some setting in the on board software but I don't know how I'd adjust it. That would also mean that the on board software is somehow allowing the pressure to drop automatically which feels unlikely. I also asked my pet mechanic but he couldn't think of anything either.

Can anyone suggest what might be going on here?

 

Temperature difference from when the tires were filled. Always fill tires at coldest part of day before any driving is done. Driving heats the tires and air inside as well.

 

As stated above, depending on your location and outside temp variation based on the season, one should expect a one PSI drop per ten degrees (F scale) drop overnight. This is Boyles law. It should impact all tires to the same degree. If not, then you have a tire with a slow leak. You should only have to do this once upon winter onset (bring the PSI up to spec levels when the tires are cold)..

 

I had thought that temperatures might be having an effect but it seems much greater than it should be. They're dropping 2 psi and we're not seeing a drop of 20 degrees. And I haven't seen this in previous years despite owning the car for the last 6 winters.

Anyway, things just got even weirder. My car was due a service so I got that done and the problem appears to have disappeared. The pressures are now reading fine. I've no idea if the mechanic did something that would have reset things but I should get a chance to quiz him in the next couple of weeks. If he can think of anything that might be relevant I'll post it back here. Otherwise I can only conclude that the fairies fixed it.

 

2 psi is nothing. For every 10 minutes you drive the pressure increases 1 psi up to a limit. Next time you take the car out for the day check the pressure when you start and watch it as you drive. It's the temperature, nothing more.

 

It hasn't done that before. The pressure doesn't change over the course of a journey for me though I can't explain why it wouldn't.

<shrug>It's academic at this point, though, as the tyres are now locked in to a healthy 35 aren't budging.