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A friend has a leaky or flat tire. We looked all over for a nail or screw or some sort of puncture on the tire and couldn’t find the problem. But, when I was adding air, I discovered that it wasn’t the tire, but the valve stem itself that was leaking on her car. It’s a surprisingly common problem that you can fix at home with a kit from your local auto parts store.

How do I diagnose a leaking valve stem?

When your tire keeps going flat, but there are no nails in it, the valve stem is likely to blame. The valve stem is the part that sticks out that you attach a pump or air hose onto. Inside, there’s a little valve that acts like a one-way door for air. But, sometimes the hinges in that tiny door get clogged or bent and start to leak.

The easiest way to spot a leak is to spray some soapy water on the valve stem. If you see bubbles forming, you have a leak. Fortunately, it’s not a huge issue. Most cars have a tubeless rubber snap-in tire valve that is easy to repair.

Can you fix a leaking valve stem?

A valve stem and tire pressure gauge
A tire pressure gauge at the valve stem of a car tire | Ben Hasty via Getty Images

The most likely culprit is probably the core itself. The core is the metal valve inside the stem. You can buy a cheap valve core removal tool kit at your local auto parts store for less than $10. The tool acts like a screwdriver for the valve and you can simply unthread it out, according to Family Handyman. The kit will come with a new valve, and you can simply screw that in. Usually, this will fix the issue.

It’s always best if you jack up the wheel, deflate the tire, and gently remove the core. It’s easy to strip the delicate threads.  If it doesn’t, you can by a tubeless rubber snap-in tire valve.

Can I fix a leaking valve stem without removing the tire?

The different parts of a car tire valve
The parts of a car tire valve | AzN8X Wikimedia Commons

If a new core doesn’t fix it, you may need to buy a rubber snap-in tire valve. A car’s rubber seals can often get corroded, or the valve can get loose and leak air. Fortunately, you can get a kit that comes with a new stem. Basically, you yank out the old valve and install a new one in your rim. The kit will have directions, but be patient and you need to use some WD-40 to loosen things up.  

The kit comes with a forcing cone and usually some grease, as well as simple instructions. You force the stem into the rim, and then pull it back out until it clicks into the rim. Kits like this cost about $40. The stems come in several sizes so find a kit with multiple valve stems and closely match up the two.

However, if your car has a TPMS, or Tire Pressure Monitoring System, you may be out of luck trying to do this at home. Many systems use a transponder on the inside of the tire that is connected to the valve. If you have a TPMS sensor, it’s best to visit a professional tire shop.

Why are my valve caps green?

A green valve cap on a truck tire
Green valve caps mean your tires are filled with nitrogen | Dan Mayfield, MotorBiscuit

Like the colors on gas cans, different colors on valve caps mean different things. My Ram truck tires are filled with nitrogen, and the dealer installed green caps to remind me of that. Generally, most valve stem caps are black. That means regular air. If yours are green, that is because your tires are filled with nitrogen. Everything else should be the same in the tire, however.