Tires often leak because of a leaky valve stem, which can release a small amount of air -- a condition that worsens in cold weather. You may notice that one of your tires loses air faster than the others but shows no visible signs of damage, such as from a nail or screw. Repairing or plugging a leaky valve stem is not always possible. The best way to fix a valve stem is to replace it.
Remove the valve cap and spray the area with window cleaner. Ensure that the window cleaner surrounds the base of the valve stem and pools up in the valve stem opening so that any leaking air will cause bubbles. Bubbles come out of the opening of a damaged valve core; bubbles also may be present around the base or along the rubber of the valve stem.
Replace the valve core by removing it with a valve core removal tool and inserting a new valve core. Tighten the core until it is firmly in place. Respray it to make sure it no longer leaks. This step applies to replacing a damaged valve core only.
Raise the vehicle with a jack and remove the lug nuts with a lug wrench. Set the lug nuts to the side. Remove the tire from vehicle and set it on a flat surface. Remove the valve core to release the air pressure.
Break the seal between the bead and the rim. Place a pry bar between the lip of the rim and the bead and apply substantial downward pressure. Push the bead down until you have sufficient room to access the other side of the valve stem.
Clip the rubber base of the valve stem to make it easier to remove. Firmly grasp the valve stem outside of the rim with pliers and pull it free. Insert a new valve stem into the hole and pull it through in the same manner until it is fully seated on the rim.
Use an air pump to inflate the tire to factory specifications. Install the valve core if it was removed using the valve core removal tool. Spray the valve with window cleaner to ensure that it no longer leaks.
Install the tire on the vehicle and tighten the lug nuts. Slowly lower the vehicle to the ground and remove the jack. Tighten the lug nuts with a lug wrench to factory-recommended torque.