CARS.COM — A slow leak in your tire may be difficult to notice at first, especially if there's no obvious damage to the rubber of the tire or nail sticking out through the hole it's just made. While it can eventually cause a flat tire, it does so gradually, unlike a very obvious blowout. This sort of issue can befall even a new tire or vehicle, but is also a very common repair that can sometimes be done at home. Home repair is not always possible, however, particularly in the case of rim damage.
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A built in tire-pressure monitor system or TPMS may eventually notify you of lower pressure in one of your car's tires as the leak causes the PSI to decrease. If your car doesn't have such a system, you might notice a change in ride quality while driving. A visual inspection when stopped may indicate that the tire is losing air (or gas, in the case of nitrogen-filled tires) and starting to go flat. Before a repair can be made, however, the source of the slow leak must be found.
If a thorough inspection of a leaking tire, which will probably require removing it from the vehicle, doesn't find a nail or puncture, the slow leak could be caused by a pinhole in the tread or sidewall. The tire might not be the problem, though. The air valve stem might have a leak and need to be replaced, or the tire bead (where it meets the wheel) might not be sealed snugly against the rim (a common problem in areas that use road salt, which can corrode the metal surface of the rim).
Soap and water, or water alone, can help find the source of a slow leak prior to any repair. Mix liquid soap with water in a spray bottle and spray all parts of the tire — tread, sidewalls, the valve stem and opening (with the cap removed), and along the rim on both sides — with the soapy water until you find a spot where bubbles start to form. That's where the air is leaking. This is easier to do with the wheel off the car, but you might be able to find the leak without removing the wheel, especially in front, where turning the steering wheel exposes the inner sidewall somewhat.
Another method to find a leak is to remove the tire and wheel from the vehicle and dunk them into a tub of water. Bubbles will form at the spot of the leak. If the tub isn't large enough to dunk the whole tire, do sections at a time.
Pinholes and small punctures in the tread causing a leak can be plugged or patched. Large punctures cannot, and minor damage to the sidewalls or shoulders (where the tread and sidewall meet) typically calls for replacement with a new tire, as well. Valve stems and cores (the tiny valve itself, inside the tube) with leaks also can be replaced.
If the slow leak is because the wheel isn't fully seated against the tire, sometimes removing the tire and applying a bead sealer can stop the leak.
Possible solutions for a leak that originates within the wheel's bead seat are to remove the tire, clean off any corrosion and apply a bead sealer before remounting the tire. Some mechanics also suggest inflating the tire with nitrogen instead of air because its molecules are larger than oxygen, potentially making them less able to slip through a small hole causing a leak. Perhaps more important, though, nitrogen contains less moisture — which will prevent rust if the wheel is made of steel (see more on this topic). Others suggest grinding or sanding off corrosion, though the time and labor required to do that might cost more than a new or used wheel — as might repeated nitrogen purchases.
When the wheel is the cause of a slow leak, it will be a judgment call as to whether it can be repaired or needs to be replaced. For example, pitting in the wheel can make the metal porous and allow air to leak out. That's likely to warrant replacement.
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