Tyre pressure increasing by itself

[Crispin]

Folks - not really a LC question but something that is baffling me and their are cleverer people than I on this forum.

My mom, in SA, has a VW Tiguan who's tyres seem to over-inflate by themselves. She'll get them check at the same pump and the same garage, 2.2 bar all round and a week later the warning light will come on in the car and they all just under 3 bar by varying amounts.

She swears blind it's the same pump at the same station
She swears blind the weather is consistent (Winter in JHB - sunny and clear for months)
After each "fix" back to 2.2 she resets the tyre pressure system.
This has happened 3 times now to her.

She is about to buy a pressure monitor so she can check it herself with a consistent baseline.

Any ideas why it might be happening? Sadly, it does not happen with the fuel tank...

Cheers,
Crispin

 

[StarCruiser]

Hi Chrispin, sounds like a faulty monitor to me. Getting a tyre pressure gauge is a good idea to give a reading to test against, kind of a calibration.

It could also be that the brakes are partly sticking on and overheating the tyre pushing the pressure up but it sounds unlikely it would get that high.

Tyre pressure gauge is the way to go IMHO.

 

[Chris]

Got to ask CP exactly what pressure does the book say they should be inflated to - warm. Tyre pressure varies a great deal (I have two TPMS) and it shows tyre temp to. On one I can actually set the set points myself. The other is fixed. Weight in the vehicle affects pressure rise more than anything else I have seen. So setting tyres to to pressure when cool (not driven far) but with an empty fuel tank and only one person in it might not trip the limit. Next week, full tank, full car and driven quite a way could be all the difference. My tyres typically start about 24 psi and reach about 30 maybe 32 psi on the run.

 

[Crispin]

Thanks chaps -

I did not ask about the full / empty tank - I shall. Knowing my mom though, she's one of those "can you just put R10 in please" each time.

As for the brakes sticking - I did think that and asked her. While not checking the disks she said she could not smell anything. My 120 had sticky brakes and I don't recall smelling them - just noticed the disk was blue. Will check when she's back.

Regarding the inaccurate sensor - I had thought that but it makes no sense. The car tripped (alerts to tyre pressure issues) and the garage says they're high so they are different to what they were to start. Something is changing.
I don't think the system can set trigger points - I think it's just a "it's not what it was when you zero'd it" type of warning. After each trip to the garage she presses and holds the button to reset it.

Chris - she does short journeys; about 6 miles to work....Actually, I wonder. Winter in JHB now. I wonder if pumping them up in the morning when the temp is -1 and air in the garage compressor tank has chilled nicely overnight. Then as it warms during the day the it expands and trips the sensors. Will ask when she pumps the tyres. Any engineers here who can work out the increase in pressure?

Until we can get a solid comparison with a gauge then we won't really know who's right and who's wrong...