What a slow puncture looks like
A slow puncture is a leak that lets air out gradually, over hours or days, rather than all at once. The usual sign is one tire that needs topping up more often than the others, a pressure warning light that returns after you have filled the tire, or a tire that looks a little lower each morning. Because it can be easy to ignore, it is a common reason for tires to run under-inflated for weeks, which damages the tire and affects handling.
It is worth separating this from normal pressure loss. All tires lose a small amount of air gradually, and cold weather lowers pressure too. If every tire is a little low after a cold spell, that is likely not a puncture. If one tire drops noticeably faster than the rest, something is probably wrong.
First steps
- Check with a gauge. Measure all four tires, cold, and compare them to the figure on the vehicle label. The method is in the article on checking pressure at home.
- Reinflate to the correct figure and note the reading.
- Recheck after a day or two. If it has dropped again by more than a small amount, you probably have a leak.
If the tire is very low, avoid driving on it. Driving on a seriously under-inflated tire can damage the sidewall in a way that makes it unsafe and unrepairable. Inflate it first, and if it will not hold, use the spare as described in the guide to changing a flat.
Common causes
- A nail, screw or other object in the tread. Sometimes the object stays in place and plugs the hole partially, which causes the slow leak.
- A leaking valve, often from a worn valve core, a damaged stem or a missing or loose cap.
- A corroded or damaged wheel rim, where the tire seals against the bead, especially on older wheels.
- A bead leak where the tire sits against the rim, sometimes caused by dirt or corrosion.
- A damaged or failing pressure sensor on cars with sensors inside the wheel.
- A cut or crack in the tire, including sidewall damage or aging cracks.
Finding the leak
You can do some of this at home.
- Inflate the tire to the correct pressure, or slightly higher if the manual allows, so the leak is easier to find.
- Look and feel carefully around the tread, shoulders and sidewall for nails, screws or cuts. Be careful of sharp objects.
- Mix soapy water in a spray bottle or bowl (dish soap and water), and apply it to the tread, valve and the rim edge. A leak will show as bubbles forming or growing.
- Mark the spot with chalk or a marker.
If you find an object stuck in the tire, leave it in place. Removing it can let the air out quickly. A tire shop can remove it and inspect the damage.
If you find no leak, a shop can check more thoroughly, including by submerging the wheel in water, which is a common and reliable method.
Can it be repaired
Whether a punctured tire can be repaired depends on where and how big the damage is. These are general guidelines, and a tire shop makes the final call.
- Puncture in the central tread area: often repairable.
- Small hole: repair standards commonly limit repairable holes to about a quarter inch (6 millimeters), though limits differ by region and maker.
- Damage to the shoulder or sidewall: generally not repairable, because these areas flex too much for a repair to hold safely.
- Tires driven while flat or very low: often not repairable, since the inside may be damaged even when the outside looks fine.
- Multiple punctures too close together, or a previous repair in the same place: may rule out another repair.
- Old, badly worn tires: often better replaced; see the article on when to replace tires based on tread depth and age.
What a good repair involves
A proper repair is done from the inside. The technician removes the tire from the wheel, inspects the inside for hidden damage, and then fills the hole with a plug and seals the inner liner with a patch (often a combined plug-and-patch). A plug pushed in from the outside alone, with no inspection, is generally treated as a temporary fix at best. Likewise, sealant from a kit, as covered in the article on spare tires and sealant kits, is a temporary measure and not a permanent repair.
A shop will usually also check the valve and rebalance the wheel if needed.
When to replace instead
Replacement is the safer choice when the damage is outside the repairable area, when the tire has been run flat, when it is close to the tread wear limit or when it is aged. Replacing two tires on the same axle together is often advised, so tread depths stay similar. Your shop can advise. Keep in mind that run-flat tires have their own repair rules, and manufacturers may advise against repair.
Do not delay
A tire that loses air slowly is a tire at risk. Keep an eye on it, top it up in the meantime, and arrange a repair or replacement soon. If a flat leaves you stuck, roadside assistance can help. Recommendations differ by tire and location, so ask a qualified technician if in doubt.