Why does one tyre keep losing pressure?

Why alloy wheels leak at the rim, how to find a slow puncture with a spray bottle, and the three causes that have nothing to do with a nail.

Difficulty: beginner
Time: 12 min
Read: 5 min
Safety: caution

Short answer

Every tyre loses a small amount of pressure naturally — a few psi over a month is normal, and cold weather makes it look worse because pressure falls roughly 1 psi for every 10°C drop. A tyre that needs topping up weekly has a fault. The usual suspects are a nail you have not found, corrosion between an alloy wheel and the tyre bead, a perished or damaged valve, or a cracked wheel. A tyre bay can find all four in a dunk tank in five minutes.

There is a persistent belief that a slow puncture means a nail, and about half the time it does. The other half is the part people struggle with, because the leak is not in the tyre at all. Alloy wheels corrode where the tyre bead seals against them, especially on cars that see winter road salt, and the resulting rough surface lets air past in a way that is completely invisible with the tyre inflated and mounted. Rubber valves perish and crack at the base. And a wheel that has taken a hard pothole hit can have a hairline crack in the rim that only leaks under load. Knowing that the leak might not be the tyre saves people from replacing a perfectly good one.

Safety

A tyre running persistently below its correct pressure heats up, wears its shoulders away and is at risk of a sudden failure at speed. Do not treat a weekly top-up as a way of living with a slow puncture. If a tyre goes from correct to visibly soft within a day, do not drive on it at motorway speed — get it looked at first.

Step by step

Establish how fast it is actually going down.Check it cold with your own gauge, note the pressure and the date, and check again after a week without touching it. A few psi in a month is normal. Several psi in a week is a fault. This measurement also tells the tyre bay how hard to look.
Rule out the weather.Tyre pressure falls by roughly 1 psi for every 10°C drop in temperature, so a car set to the right pressure in September can be several psi down on a frosty morning with nothing wrong at all. Compare against the other three tyres, which are experiencing the same weather.
Look carefully at the tread with the wheel turned.Turn the steering to full lock to expose the front tyres, and go over the tread in good light and by feel. Screws and nails often sit flush and are easy to miss. Check the inner shoulder too, which is where debris tends to lodge and where nobody looks.
Try a soapy water test.Mix washing-up liquid with water in a spray bottle and wet the tread, the sidewall, the valve and, importantly, the join between the tyre and the rim all the way round. A leak blows a cluster of small persistent bubbles. It is the same test a tyre bay does, and it costs nothing.
Check the valve specifically.Spray around the base of the valve stem where it passes through the wheel, and around the top with the cap off. Rubber valves perish with age and crack at the base; metal valves on cars with pressure sensors have a seal and a nut that can loosen or corrode. Valves are cheap and are usually replaced with the tyre.
Suspect the rim if the leak is around the bead.Bubbles appearing all along the join between tyre and wheel means corrosion on the sealing face. The fix is to break the tyre off the rim, clean the seat back to clean metal, and refit — a routine job at a tyre bay and much cheaper than a wheel.
Consider a cracked wheel after a pothole.If the slow leak started immediately after an impact, and no puncture can be found, the rim may be cracked or distorted. This is a wheel replacement or a specialist repair, and it is worth checking the alignment at the same time.
Do not just keep topping it up.A tyre that runs several psi low for months wears its shoulders out, uses more fuel and runs hot. The diagnosis is inexpensive and the repair is usually cheaper still.

Common mistakes

Pulling out the nail before getting to a tyre bay — The nail is partly sealing the hole. Removing it in a car park usually converts a slow leak into a flat tyre and a recovery call.
Topping up weekly and calling it managed — The tyre spends most of its life under-inflated, which destroys its shoulders, raises fuel consumption and increases the risk of a heat-related failure at speed.

If it doesn't work

All four tyres down a few psi at once

Cause: Cold weather, or simply time since they were last set — Fix: Normal. Set them all to the placard pressures cold. The catalogue's guide on the pressure warning light in cold weather covers the temperature effect.

Leak found at the rim on an older alloy wheel

Cause: Corrosion on the bead seat, common on salted winter roads — Fix: The tyre comes off, the seat is cleaned and sealed, and the tyre goes back on. It is a routine tyre bay job and does not need a new wheel.

Tyre pressure light on but all pressures check out

Cause: A failed sensor or a flat sensor battery — they last a number of years and are not replaceable separately on most cars — Fix: Have the sensor read with a TPMS tool. Sensors are typically replaced with the tyre once they reach the end of their battery life.

Only loses pressure when the car is parked for a week

Cause: A very small leak, often a valve or bead — Fix: Still worth fixing. Soapy water on the valve and bead is the most likely to find it; a dunk tank at a tyre bay will find it for certain.

Loses pressure fast when driven, holds when parked

Cause: A crack or a damaged bead that opens under load and flex — Fix: This one needs the tyre off the wheel to inspect, and often means replacement. Do not drive at speed on it while you decide.

Questions people ask

How much pressure loss is normal?

A tyre naturally permeates a small amount of air through the rubber — a couple of psi a month is unremarkable, which is why monthly checks are recommended. Anything faster than that is a leak worth finding.

Should I use nitrogen?

Nitrogen molecules permeate slightly more slowly than oxygen, so pressures hold a little better, and it is common in aviation and motorsport where temperature swings are extreme. On a road car the benefit is small, air is already about 78 per cent nitrogen, and it is no substitute for checking pressures. It certainly will not fix a leak.

Can a tyre pressure sensor cause a slow leak?

Yes. On cars with sensors fitted inside the wheel, the sensor is mounted to the valve and its seal can fail or its retaining nut can corrode. That is one of the specific things to have checked when no puncture can be found.