Tyre Suddenly Flat......Why?

[Slowboathome]

Bit of an odd one this.....

One of my tyres is losing a few lbs of pressure every 48 hours or so. Doesn't seem to lose pressure when driving.

But it will suddenly lose all pressure when parked overnight (happened at 2 different addresses in the last week).

I've booked it in at a local tyre place tomorrow, but I wondered if anyone recognises this phenomenon?

Why is it gradually losing a little pressure then suddenly going flat?

[bobtail4x4]

I had this once, turned out it went flat faster when below zero, I guess the slight leak opened more when it froze.

[zetec]

If you are parked with the hole at the point where tyres meets surface, the pressure is maybe enough to keep the tyres inflated? Parking with the hole at the top of the tyres means air escapes quicker?

Maybe?

[Notso]

When you drive the air heats up and expands so the tyre doesn't appear to go down so much. It's basic physics and we covered that that when I was at school not so long ago.

[CABC]

I've had two leaky valves that leaked erratically. a drive sometimes 'resolved' it. for a while..

[Slowboathome]

zetec said:

If you are parked with the hole at the point where tyres meets surface, the pressure is maybe enough to keep the tyres inflated? Parking with the hole at the top of the tyres means air escapes quicker?

Maybe?

Cheers for the suggestions - this might be what's going on.

It is a bit of a mystery: it's been parked overnight for 2 nights without noticeable loss of pressure then on the 3rd night it went completely flat.

Odd.

[DonkeyApple]

Could be several reasons. If quite low profile then they have a tendency to look fine right until the last few pounds of pressure then suddenly look flat. As above, could be a hole in the tyre where the leak rate is dependent on where it is when parked. My initial assumption was that you have a crack near the rim which leaks more when parked at a certain rotation.

I've had random leaks like this on low profile alloys where both the wheel and tyre were fine but reseating the tyre solved the problem!

[larrylamb11]

Most likely is corrosion of the rim and it's seeping air between the bead of the tyre and the rim.... It'll suddenly get worse when you park with the wheel rotated to where the bit that's seeping on the rim is under deflection and the leak worsens.
Take it to tyre shop, get tyre de-mounted, rim cleaned and refitted. Don't assume the tyre is punctured as there may be nothing wrong with it - any tyre shop worth going to will establish where the air is escaping before recommending a remedy. Soapy solution sprayed around the rim will tell you.
This is usually a problem that is worse in the winter as the road salt corrodes the alloy.

[carl_w]

DonkeyApple said:

If quite low profile then they have a tendency to look fine right until the last few pounds of pressure then suddenly look flat.

I once had the tyre pressure warning come up on my Z4MC which aren't really low profile tyres compared to some more modern stuff (225/45R18 front, 255/40R18 rear) but when I pulled over into a convenient, well-lit petrol station I couldn't work out which one was flat from a visual inspection.

[Slowboathome]

larrylamb11 said:

Most likely is corrosion of the rim and it's seeping air between the bead of the tyre and the rim.... It'll suddenly get worse when you park with the wheel rotated to where the bit that's seeping on the rim is under deflection and the leak worsens.
Take it to tyre shop, get tyre de-mounted, rim cleaned and refitted. Don't assume the tyre is punctured as there may be nothing wrong with it - any tyre shop worth going to will establish where the air is escaping before recommending a remedy. Soapy solution sprayed around the rim will tell you.
This is usually a problem that is worse in the winter as the road salt corrodes the alloy.

Cheers. It's the second tyre that has gone on the same wheel in the past year, so maybe it is something to do with the rim.

I'll talk it through with the tyre place and make sure they don't just bung another one on.

[SkodaIan]

It could be the valve. They require a certain amount of pressure to hold them shut.

I had a similar issue with a mountain bike tyre where it would be fine for ages and then suddenly go completely flat.

The valve problem could be as simple as a bit of grit/dirt in it. I fixed the bike tyre by over inflating it a bit (nothing extreme) and then rattled the valve open and shut whilst letting air out of the tyre. That was about 2 years ago and it's held air for months at a time since so it must have solved the problem.

[HappyMidget]

Definitely checks for a cracked rim, had similar on my car, local welder sorted it for £50. Just had to take the wheel to KwikFit to remove and then remount the tyre as he did not have the machinery to do it himself.

[stevemcs]

Cracks or flat spots usually, normally if you park with the flat spot at the bottom it breaks the seal and leaks, corrosion is the popular too, most common cars for cracked rims are BMW's Mercs and Teslas.

[Emily's dad]

I had similar on one of my cars recently.
New dust cap on the valve solved it.

[vikingaero]

Emily's dad said:

I had similar on one of my cars recently.
New dust cap on the valve solved it.

Dust caps are for keeping debris out of the valve stem. Whilst they can hold some pressure, that's not what they were designed for and you should really look at the underlying issue!

[Emily's dad]

vikingaero said:

Emily's dad said:

I had similar on one of my cars recently.
New dust cap on the valve solved it.

Dust caps are for keeping debris out of the valve stem. Whilst they can hold some pressure, that's not what they were designed for and you should really look at the underlying issue!

The issue was resolved after replacing a split dust cap. It may well have been that the split dust cap allowed some debris into the valve, but pumping the tyre up and fitting a new dust cap solved my issue.

Edited by Emily's dad on Thursday 30th December 09:03

[Slowboathome]

Just to (hopefully) put this to bed.....

Tyre place reckoned the rim hadn't been properly cleaned when they swapped it onto winter tyres in November so it hadn't sealed properly. Cleaned it up and refitted at no charge.

Fingers crossed that's sorted it.