Tire keeps losing air but there is no nail in it

One tire needs topping up every week or two. You have been over the tread twice and there is nothing in it. This is a common and genuinely annoying problem, and in most cases the tire itself is fine.

There are four usual causes, and they are cheap in a different order than people expect.

The valve stem

The most likely answer and the cheapest fix on the entire car.

The valve stem is the short rubber or metal stalk you put air through. Rubber ones perish with age, road salt and ozone, and they crack at the base where they pass through the wheel. Metal ones, which carry the TPMS sensor on most modern cars, have a rubber grommet and a core that can both fail.

How to check: put a little soapy water around the base of the stem and on the valve itself with the cap off. Bubbles mean it is leaking. You can often hear it in a quiet garage.

A stem is replaced when the tire is off the wheel, so it takes the same visit as any tire job.

The bead seal

The bead is where the edge of the tire clamps against the wheel rim. It seals by pressure alone, and anything between the two surfaces breaks the seal: corrosion on an alloy wheel, road grit, or a dented rim from a pothole.

How to check: soapy water right along the line where rubber meets metal, all the way round, on both the outer and inner faces of the wheel. The inner face is where most of these hide, which is why people miss them.

The fix is to break the tire down off the rim, clean the mating surface back to bare metal, and reseat it. It does not need a new tire.

A corroded or cracked wheel

Older alloy wheels corrode from the inside, particularly in places that salt the roads. Steel wheels rust. Either can leak through the metal itself or lose the bead seal because the surface has pitted.

This one is visible only once the tire is off. If the wheel is cracked, usually from a pothole impact, it needs replacing rather than sealing.

A puncture you cannot see

Not every puncture leaves something behind. A nail can work its way out and leave a hole that only leaks under load, or a thorn can make a pinhole that closes when the car is parked and opens as the tire flexes.

These are found by submerging the tire, which means taking it off the wheel. Soapy water on the tread will sometimes reveal it, but not reliably.

How to narrow it down before anyone arrives

Note how fast it goes down. A few PSI a week suggests a stem or a bead. Overnight suggests a puncture or a badly failed seal.

Note whether it is worse in cold weather. Air contracts as it cools and a tire loses roughly 1 PSI for every 10 degrees Fahrenheit, so a tire that seems to lose air only in winter may not be leaking at all. If it comes back up on a warm afternoon, that is temperature, not a leak.

Note whether it is always the same tire. If it moves around, it is probably temperature or an inaccurate gauge.

Why this is worth fixing rather than topping up

A tire run persistently underinflated wears at both shoulders and not in the middle, so you lose tread life across the whole tire. It also runs hotter, and heat is what destroys tires. Topping up weekly costs nothing in money and quite a lot in tire.

What a fix involves

All four causes need the tire off the wheel, which is a single visit. We take the wheel off, break the tire down, find the actual cause rather than guessing, and fix it: a new stem, a cleaned and reseated bead, or a proper plug-and-patch repair if there is a hidden puncture. If the wheel itself is cracked we tell you, because that is not something to seal and send away.

Questions people ask

• Can I just use a sealant from a can? For a genuine emergency, briefly. Sealant does not fix a stem or a bead, and it makes the inside of the tire messy enough that some shops will refuse to repair it afterwards.
• It only loses air in winter. Is it leaking? Possibly not. Cold air contracts. If it returns on a warm day and holds, that is physics rather than a leak. If it keeps dropping regardless, it is a leak.
• The TPMS light comes on and then goes off. Consistent with a slow leak crossing the warning threshold as the temperature moves. Worth finding rather than ignoring.
• Is a leaking valve stem dangerous? It can fail suddenly rather than slowly, which at speed is a blowout. It is also one of the cheapest things on the car to replace.
• Do I need a new tire for any of these? Usually not. Three of the four causes are the wheel or the stem, not the tire.